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.
• Requested review of a "hold as draft" revision to kick it out of draft state.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T2543

Differential Revision: https://secure.phabricator.com/D18737