R1:dc35ce79e484

Unprototype "Draft" mode in Differential

Summary: Fixes T2543. This mode has been a stable prototype for a very long time now; promote it so "--draft" can promote out of "experimental" in Arcanist.

Test Plan: See T2543 for discussion.

Maniphest Tasks: T2543

Differential Revision: https://secure.phabricator.com/D20983