Add table for project alert http integrations

What does this MR do?

Adds a table & preliminary model for HTTP Integrations for alerts!

Related issue: #255511 (closed) from epic &4442 (closed)

Intended use:

• An HTTP Integration will allow an endpoint to be configured which can receive alert notifications from any source as long is it provides the appropriate token.
• New integrations will be have URLs formatted according to #223778 (closed). An expected url will look like: https://gitlab.com/group/project/alerts/notify/<name>/<endpoint_identifier>.json; We will lookup the active integration by the endpoint identifier, allowing the name portion mostly to be used for user-readability and organization.
• Existing integrations from AlertsService & AlertsServiceData will eventually be migrated into HTTP Integrations. These are expected not to have a name or endpoint_identifier, and they will continue to accept alerts at https://gitlab.com/group/project/alerts/notify.json, as they do now.
• token assignment is air-lifted from AlertsService & AlertsServiceData

----- Check out this keen supplemental database info! -----

Up migrations

$ rails db:migrate
== 20200928203531 CreateAlertManagementHttpIntegrations: migrating ============
-- create_table(:alert_management_http_integrations, {:if_not_exists=>true})
   -> 0.0214s
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0002s
-- execute("ALTER TABLE alert_management_http_integrations\nADD CONSTRAINT check_f68577c4af\nCHECK ( char_length(encrypted_token) <= 255 )\nNOT VALID;\n")
   -> 0.0012s
-- current_schema()
   -> 0.0002s
-- execute("SET statement_timeout TO 0")
   -> 0.0001s
-- execute("ALTER TABLE alert_management_http_integrations VALIDATE CONSTRAINT check_f68577c4af;")
   -> 0.0005s
-- execute("RESET ALL")
   -> 0.0002s
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0002s
-- execute("ALTER TABLE alert_management_http_integrations\nADD CONSTRAINT check_286943b636\nCHECK ( char_length(encrypted_token_iv) <= 255 )\nNOT VALID;\n")
   -> 0.0008s
-- current_schema()
   -> 0.0002s
-- execute("ALTER TABLE alert_management_http_integrations VALIDATE CONSTRAINT check_286943b636;")
   -> 0.0004s
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0002s
-- execute("ALTER TABLE alert_management_http_integrations\nADD CONSTRAINT check_e270820180\nCHECK ( char_length(endpoint_identifier) <= 255 )\nNOT VALID;\n")
   -> 0.0004s
-- current_schema()
   -> 0.0001s
-- execute("ALTER TABLE alert_management_http_integrations VALIDATE CONSTRAINT check_e270820180;")
   -> 0.0008s
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0002s
-- execute("ALTER TABLE alert_management_http_integrations\nADD CONSTRAINT check_392143ccf4\nCHECK ( char_length(name) <= 255 )\nNOT VALID;\n")
   -> 0.0003s
-- current_schema()
   -> 0.0001s
-- execute("ALTER TABLE alert_management_http_integrations VALIDATE CONSTRAINT check_392143ccf4;")
   -> 0.0004s
-- add_index(:alert_management_http_integrations, [:active, :project_id, :endpoint_identifier], {:unique=>true, :name=>"index_http_integrations_on_active_and_project_and_endpoint", :where=>"active"})
   -> 0.0082s
== 20200928203531 CreateAlertManagementHttpIntegrations: migrated (0.0587s) ===

== 20200928210524 AddHttpIntegrationsProjectForeignKey: migrating =============
-- add_foreign_key(:alert_management_http_integrations, :projects, {:column=>:project_id, :on_delete=>:cascade})
   -> 0.0289s
== 20200928210524 AddHttpIntegrationsProjectForeignKey: migrated (0.0304s) ====

$ bin/rails dbconsole
psql (11.9)
Type "help" for help.

gitlabhq_development=# \d alert_management_http_integrations
                                           Table "public.alert_management_http_integrations"
       Column        |           Type           | Collation | Nullable |                            Default                             
---------------------+--------------------------+-----------+----------+----------------------------------------------------------------
 id                  | bigint                   |           | not null | nextval('alert_management_http_integrations_id_seq'::regclass)
 created_at          | timestamp with time zone |           | not null | 
 updated_at          | timestamp with time zone |           | not null | 
 project_id          | bigint                   |           | not null | 
 active              | boolean                  |           | not null | false
 encrypted_token     | text                     |           | not null | 
 encrypted_token_iv  | text                     |           | not null | 
 endpoint_identifier | text                     |           | not null | 
 name                | text                     |           | not null | 
Indexes:
    "alert_management_http_integrations_pkey" PRIMARY KEY, btree (id)
    "index_http_integrations_on_active_and_project_and_endpoint" UNIQUE, btree (active, project_id, endpoint_identifier) WHERE active
    "index_alert_management_http_integrations_on_project_id" btree (project_id)
Check constraints:
    "check_286943b636" CHECK (char_length(encrypted_token_iv) <= 255)
    "check_392143ccf4" CHECK (char_length(name) <= 255)
    "check_e270820180" CHECK (char_length(endpoint_identifier) <= 255)
    "check_f68577c4af" CHECK (char_length(encrypted_token) <= 255)
Foreign-key constraints:
    "fk_rails_bec49f52cc" FOREIGN KEY (project_id) REFERENCES projects(id) ON DELETE CASCADE

Down migration: add project foreign key

$ rails db:migrate:down VERSION=20200928210524
== 20200928210524 AddHttpIntegrationsProjectForeignKey: reverting =============
-- remove_foreign_key(:alert_management_http_integrations, {:column=>:project_id})
   -> 0.0069s
== 20200928210524 AddHttpIntegrationsProjectForeignKey: reverted (0.0150s) ====

$ bin/rails dbconsole
psql (11.9)
Type "help" for help.

gitlabhq_development=# \d alert_management_http_integrations
                                           Table "public.alert_management_http_integrations"
       Column        |           Type           | Collation | Nullable |                            Default                             
---------------------+--------------------------+-----------+----------+----------------------------------------------------------------
 id                  | bigint                   |           | not null | nextval('alert_management_http_integrations_id_seq'::regclass)
 created_at          | timestamp with time zone |           | not null | 
 updated_at          | timestamp with time zone |           | not null | 
 project_id          | bigint                   |           | not null | 
 active              | boolean                  |           | not null | false
 encrypted_token     | text                     |           | not null | 
 encrypted_token_iv  | text                     |           | not null | 
 endpoint_identifier | text                     |           | not null | 
 name                | text                     |           | not null | 
Indexes:
    "alert_management_http_integrations_pkey" PRIMARY KEY, btree (id)
    "index_http_integrations_on_active_and_project_and_endpoint" UNIQUE, btree (active, project_id, endpoint_identifier) WHERE active
    "index_alert_management_http_integrations_on_project_id" btree (project_id)
Check constraints:
    "check_286943b636" CHECK (char_length(encrypted_token_iv) <= 255)
    "check_392143ccf4" CHECK (char_length(name) <= 255)
    "check_e270820180" CHECK (char_length(endpoint_identifier) <= 255)
    "check_f68577c4af" CHECK (char_length(encrypted_token) <= 255)

Down migration: add integrations table

$ rails db:migrate:down VERSION=20200928203531
== 20200928203531 CreateAlertManagementHttpIntegrations: reverting ============
-- drop_table(:alert_management_http_integrations)
   -> 0.0142s
== 20200928203531 CreateAlertManagementHttpIntegrations: reverted (0.0142s) ===

$ bin/rails dbconsole
psql (11.9)
Type "help" for help.

gitlabhq_development=# \d alert_management_http_integrations
Did not find any relation named "alert_management_http_integrations".

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• Changelog entry
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• Style guides
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