Add and expose Jira app OAuth application ID

What does this MR do and why?

This adds a new application setting, so users can enter the ID of the OAuth application they created to use the GitLab for Jira app on self-managed.

GitLab.com will serve as a proxy for the app. This means it will need to fetch the application ID from the self-managed instance in order to initialize the OAuth login flow. Therefor, the application ID setting is publicly exposed.

Once the feature is done, the workflow will be like this:

1. User installs the GitLab for Jira app
2. User chooses self-managed and enters the GitLab instance URL like in this screenshot
3. Frontend calls http://gl-instance-url.com/-/jira_connect/oauth_application_id and fetches the application ID
4. Frontend uses the application ID to initialize an OAuth flow, log in the user, and fetch a user token to make API requests on the instance.

This MR does not include documentation or API changes because it is behind the jira_connect_oauth_self_managed feature flag and the jira_connect_oauth feature flag

Related issue: #353328 (closed) and #338459 (closed)

Database

This adds two new migrations:

1. Add jira_connect_application_key to application_settings
2. Add text limit for jira_connect_application_key

Migration up:

== 20220513114706 AddJiraConnectApplicationIdApplicationSetting: migrating ====
-- add_column(:application_settings, :jira_connect_application_key, :text)
   -> 0.0097s
== 20220513114706 AddJiraConnectApplicationIdApplicationSetting: migrated (0.0098s) 

== 20220513114850 AddTextLimitToJiraConnectApplicationIdApplicationSetting: migrating 
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0030s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE application_settings\nADD CONSTRAINT check_e2dd6e290a\nCHECK ( char_length(jira_connect_application_key) <= 255 )\nNOT VALID;\n")
   -> 0.0124s
-- current_schema()
   -> 0.0061s
-- execute("SET statement_timeout TO 0")
   -> 0.0014s
-- execute("ALTER TABLE application_settings VALIDATE CONSTRAINT check_e2dd6e290a;")
   -> 0.0265s
-- execute("RESET statement_timeout")
   -> 0.0013s
== 20220513114850 AddTextLimitToJiraConnectApplicationIdApplicationSetting: migrated (0.1005s) 

Migration down:

== 20220513114850 AddTextLimitToJiraConnectApplicationIdApplicationSetting: reverting 
-- transaction_open?()
   -> 0.0000s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE application_settings\nDROP CONSTRAINT IF EXISTS check_e2dd6e290a\n")
   -> 0.0056s
== 20220513114850 AddTextLimitToJiraConnectApplicationIdApplicationSetting: reverted (0.0485s) 

== 20220513114706 AddJiraConnectApplicationIdApplicationSetting: reverting ====
-- remove_column(:application_settings, :jira_connect_application_key, :text)
   -> 0.0058s
== 20220513114706 AddJiraConnectApplicationIdApplicationSetting: reverted (0.0144s) 

Screenshots or screen recordings

See &5650 (closed)

How to set up and validate locally

1. On rails console, run Feature.enable(:jira_connect_oauth_self_managed)
2. Visit http://localhost:3000/admin/application_settings/general
3. Scroll down and expand the GitLab for Jira App section
4. Enter any value
5. Visit http://localhost:3000/-/jira_connect/oauth_application_id
6. See the value you entered

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.