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.

Tested in Firefox, Chrome, Safari, IE11, Edge.

Migrated from !12493 (closed) because I had to change the branch name.

Related MR in CE: gitlab-org/gitlab-ce!29626

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