WIP: Conform related issues/MRs card to design docs

What does this MR do?

Makes the related issues and related MRs cards conform to the design docs (mostly). Specifically, it fixes gitlab-org/gitlab-ce#61357 and gitlab-org/gitlab-ce#59594.

There's still a small bug that I'm not sure can be resolved in CSS. See the discussion for more.

Tested in Firefox, Chrome, and Safari. IE and Edge coming soon.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database 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