mr: merge when the gates pass #289

mr merge <n> --when-ready queues the merge and performs it once MergeGates passes: required checks, approvals, codeowners, resolved threads.

• Evaluated when a status is set, a review is submitted, a thread is resolved, and on push to the source branch.
• A push to the source branch after queueing keeps the request queued; the new head must pass the gates again.
• The server cannot rebase on a require-signed repository (the result would be unsigned). If the branch falls behind, the request stays queued and the MR says it needs a rebase.
• mr merge <n> --cancel dequeues. The MR page shows the queued state and who queued it, with a web button for both.
• The merge is attributed to the user who queued it and checked against their rights at merge time.