Allow draft revisions to be commandeered

Summary:
See PHI457. There's no real reason not to allow this, it just wasn't clear if it was useful. See D18626.

An install had a user arc diff and then sprint out the door to take a very long vacation before the builds finished. One failed, so the revision is stuck as a draft forever. This seems like a reasonable motivation for allowing "Commandeer".

Test Plan: Successfully commandeered a draft.

Differential Revision: https://secure.phabricator.com/D19228