Resolve "Specify that an MR must be merged after another MR"

What does this MR do?

This extracts the related issuable input into it's own component so it can be used for the blocking MR input here https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13506

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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

Part of #9688 (closed)