FE display for "Specify that an MR must be merged after another MR"

What does this MR do?

This merge requests displays blocking merge requests on the "Show" page for a merge request. It does not collect the data or allow users to input it yet. However, it will not show if none exist.

CE Backport: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29095

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)