Start revisions in "Draft" if prototypes are enabled

Summary: Ref T2543. This is a less ambitious version of the rule in D18628, which I backed off from, since I think this probably still has a fair number of loose ends to tie up.

Test Plan: Created a revision locally.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T2543

Differential Revision: https://secure.phabricator.com/D18713