Merge request branch selection filtering

Proposal

Filter or sort the branch selection popups (especially on the 'create merge request' pages), so recently used branches / protected branches are prioritised.

This is motivated by having a project with a large number of old/inactive branches which cannot be deleted for other reasons. Eg we have historical release/old.version branches going back a decade, but never want to target those for a MR.

Another option would be a way to mark branches as 'archived' or similar without deleting them, so they are ignored in the UI.