Discovery: Add Jira ticket association requirement to Merge Requests

Problem to solve

Currently there is no way to block a merge request from being merged if it lacks a Jira ticket association.

Related issues

This issue is part of epic to improve the Jira <> Merge Request experience &4637.

Related issues and suggested order of implementation:

1. Add a project-level toggle to require Jira association in Merge Requests #268294 (closed)
2. This issue
3. Add a button in Merge Requests to add Jira issue to the title or description #268293
4. Suggest a Jira issue when editing a Merge Request title or description #268296 (closed)

Proposal

If a MR lacks a Jira association:

1. If the feature flag is enabled
2. If the project has Jira integration
3. Block the merge button
4. Add a message next to the merge button to explain how to resolve the issue

Implementation

• backend Backend support for blocking MR being merged without JIRA issue association
• frontend Frontend support for blocking MR being merged without JIRA issue association
• documentation Add documentation for blocking MR being merged without JIRA issue association