Implementation: Frontend support for blocking MR being merged without JIRA issue association

Why are we doing this work

Add frontend support for blocking MR being merged without JIRA issue association.

Relevant links

Non-functional requirements

• Documentation: NA
• Feature flag: issue_association_on_merge_request
• Performance:
• Testing: Make sure MR mergability behaves with and without feature flag

Proposal / Design

Note: The Edit merge request button will be added in #268293.

Implementation plan

• Get the Jira issue required to merge data from the backend #280765 (closed) and into the merge request store
• Add a new merge state for the Jira requirement
• Add a new state widget component that shows the disabled Merge button and message