Store reason for resource state events

What does this MR do?

We need to track the reason for state change events so that we can generate the correct system note.

This MR stores extra information about the state change event.

Migrations

Up

> bundle exec rails db:migrate:up VERSION=20200524104346
bundle exec rails db:migrate:up VERSION=20200615141554
bundle exec rails db:migrate:up VERSION=20200623073431
== 20200524104346 AddSourceToResourceStateEvent: migrating ====================
-- column_exists?(:resource_state_events, :source_commit)
   -> 0.0023s
-- add_column(:resource_state_events, :source_commit, :text)
   -> 0.0022s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE resource_state_events\nADD CONSTRAINT check_f0bcfaa3a2\nCHECK ( char_length(source_commit) <= 40 )\nNOT VALID;\n")
   -> 0.0005s
-- execute("ALTER TABLE resource_state_events VALIDATE CONSTRAINT check_f0bcfaa3a2;")
   -> 0.0072s
== 20200524104346 AddSourceToResourceStateEvent: migrated (0.0180s) ===========

== 20200615141554 AddClosedByFieldsToResourceStateEvents: migrating ===========
-- add_column(:resource_state_events, :close_after_error_tracking_resolve, :boolean, {:default=>false})
   -> 0.0884s
-- add_column(:resource_state_events, :close_auto_resolve_prometheus_alert, :boolean, {:default=>false})
   -> 0.0843s
== 20200615141554 AddClosedByFieldsToResourceStateEvents: migrated (0.1728s) ==

== 20200623073431 AddSourceMergeRequestIdToResourceStateEvents: migrating =====
-- column_exists?(:resource_state_events, :source_merge_request_id)
   -> 0.0023s
-- add_column(:resource_state_events, :source_merge_request_id, :integer)
   -> 0.0098s
-- index_exists?(:resource_state_events, :source_merge_request_id, {:name=>"index_resource_state_events_on_source_merge_request_id"})
   -> 0.0028s
-- add_index(:resource_state_events, :source_merge_request_id, {:name=>"index_resource_state_events_on_source_merge_request_id"})
   -> 0.0138s
-- foreign_keys(:resource_state_events)
   -> 0.0026s
-- add_foreign_key(:resource_state_events, :merge_requests, {:column=>:source_merge_request_id, :on_delete=>:nullify})
   -> 0.0144s
== 20200623073431 AddSourceMergeRequestIdToResourceStateEvents: migrated (0.0511s) 

Down

> bundle exec rails db:migrate:down VERSION=20200623073431
bundle exec rails db:migrate:down VERSION=20200615141554
bundle exec rails db:migrate:down VERSION=20200524104346
== 20200623073431 AddSourceMergeRequestIdToResourceStateEvents: reverting =====
-- remove_column(:resource_state_events, :source_merge_request_id)
   -> 0.0032s
== 20200623073431 AddSourceMergeRequestIdToResourceStateEvents: reverted (0.0102s) 

== 20200615141554 AddClosedByFieldsToResourceStateEvents: reverting ===========
-- remove_column(:resource_state_events, :close_auto_resolve_prometheus_alert, :boolean)
   -> 0.0012s
-- remove_column(:resource_state_events, :close_after_error_tracking_resolve, :boolean)
   -> 0.0008s
== 20200615141554 AddClosedByFieldsToResourceStateEvents: reverted (0.0021s) ==

== 20200524104346 AddSourceToResourceStateEvent: reverting ====================
-- remove_column(:resource_state_events, :source_commit)
   -> 0.0025s
== 20200524104346 AddSourceToResourceStateEvent: reverted (0.0026s) ===========

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team