Allow sorting by most or least comments on Issues and Merge Requests - continued

What does this MR do?

Follow up on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3395. It allows to sort Issues and Merge Requests by most or least comments.

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

Performance implications.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #12824 (moved).