R1:28cec2f8a2bf

Allow revisions to be held as drafts, even after builds finish

Summary:
Ref T2543. Instead of autosubmitting revisions to "Needs Review" when builds finish, allow them to be held in "Draft" indefinitely.

There's currently no UI for this. I plan to just expose it as `arc diff --draft` for now, in a followup change.

Test Plan:
- Created a revision (via Conduit) with "hold as draft", saw it hold as draft after builds finished.
- Created a revision (normally), saw it autosubmit after builds finished.