add toggle to override Approvals per-MR

What does this MR do?

Add option to disable overriding approvers on individual MRs

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

• migrations

Why was this MR needed?

Deliverable

Screenshots (if relevant)

Settings page:

MR Edit page (override enabled)

MR Edit page (override disabled)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• 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 #582 (closed)