WIP: Display blocking MRs when viewing a merge request

What does this MR do?

This shows a list of blocking MRs in the MR widget if they exist. It will be "resolved" if and only if all blocking MRs are "merged" (Not closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Style guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Related to #9688 (closed)