Add a project-level setting to require Jira association in Merge Requests (FE)

Problem to solve

Currently, there is no way to enable or disable that Merge Requests require an associated Jira issue.

Related issues

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

Related issues and suggested order of implementation:

1. This issue
2. Add Jira ticket association requirement to Merge Requests #268292 (closed)
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

1. Add a new toggle to Merge checks under Project settings
2. Only show if the feature flag is enabled and the project has Jira integration

Implementation

• backend Introduce feature flag issue_association_on_merge_request in #297644 (closed)
• database Add a new field named prevent_merge_without_jira_issue on project_settings table in #297644 (closed)
• backend & frontend Add ability to update Jira issue merge check via the project settings page

Technical notes

• frontend The Merge Checks form is in haml template