Part 1/2: Add self-managed sign in flow to Jira Connect App

What does this MR do and why?

Towards #356361 (closed)

This MR refactors relevant parts of the Jira Connect App so that we can develop the self-managed flow behind a feature flag.

The actual self-managed sign in UI will be delivered in the next MR (MR 2/2) - this approach was taken to keep MRs small.

This commit introduces a new feature flag: jira_connect_oauth_self_managed

How to set up and validate locally

Validate no regressions

We first want to ensure there are no regressions to the existing experience (with all feature flags disabled).

To do that, set up the Jira Connect app using this guide: https://docs.gitlab.com/ee/development/integrations/jira_connect.html#install-the-app-in-jira

Validate the feature flag-enabled state

Follow the below steps to test the feature-flag-enabled state:

1. Set up Jira and the GitLab for Jira app: https://docs.gitlab.com/ee/development/integrations/jira_connect.html#install-the-app-in-jira

2. Enable the jira_connect_oauth and jira_connect_oauth_self_managed feature flags.

3. Refresh the Jira Connect app and observe. You should get a blank page (remember: the UI is coming in the next MR).

   

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.