Draft: Jira Connect OAuth full context

What does this MR do?

This MR shows the full context of the changes regarding #284211 (closed)

We decided to split the work up into two MRs:

• frontend !67949 (closed)
• backend !67895 (closed)

How to setup and validate locally (strongly suggested)

1. Start a Gitpod and open the rails console bundle exec rails console
2. Enable the feature flag Feature.enable(:jira_connect_oauth)
3. On your GitLab instance, go to admin -> applications
4. Create a new app with, Name: Jira Connect, Redirect URI: YOUR_GITPOD_INSTANCE/-/jira_connect/oauth_callbacks, Scopes: api, Trusted: No, Confidential: No
5. Copy Application ID
6. Go to gitpod.io/variables
7. Create a new variable with name JIRA_CONNECT_OAUTH_CLIENT_ID, scope */* and paste the Application ID as value.

~"group::ecosystem" backend frontend ~"Jira::ConnectApp"