Generalize detail page headers (Issues, Merge Requests, Snippets)

What does this MR do?

• Cleanup floats after we moved to flex headers in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15192
• Generalize selectors for usage across issuables, snippets, and milestones
• Fix snippet actions not being aligned to the right, https://gitlab.com/gitlab-org/gitlab-ce/issues/40543

.	Before	After
Issues
Merge Requests
Snippets
Milestone

Are there points in the code the reviewer needs to double check?

The snippet action alignment bug, https://gitlab.com/gitlab-org/gitlab-ce/issues/40543, was introduced by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15192 and also left behind a lot a float styles after moving to flex.

We use the .detail-page-header on Issues, Merge Requests, Snippets, and Milestone pages

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #40543 (closed)