When the last rejecting reviewer resigns from a revision, return it to "Needs Review"

Summary:
Ref T13216. Fixes T12920. See PHI911. If you reject a revision and then resign from it, it stays in "Needs Revision".

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.

Test Plan:

• As the only reviewer: requested changes to a revision, then resigned.
• Before: revision stays in "Needs Revision".
• After: revision moves back to "Needs Review".

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13216, T12920

Differential Revision: https://secure.phabricator.com/D19840