When the last rejecting reviewer resigns from a revision, return it to "Needs Review"
There's some arguable motivation for this, but it's inconsistent with how "Accept" works (if the last accepting reviewer resigns, we kick you out of "Accepted"). Make it consistent.
- As the only reviewer: requested changes to a revision, then resigned.
- Before: revision stays in "Needs Revision".
- After: revision moves back to "Needs Review".
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D19840