Downgrade accepts on "request changes", and make sticky accepts optional

Summary:
Fixes T3202. This fixes a couple of workflow issues:

  - Accepted Revision -> Request Review. Currently this stays "accepted" due to sticky rules being too aggressive, but should transition to "needs review".
  - Accepted Revision -> Plan Changes -> Request Review. Currently this stays "accepted". I think this behavior is correct, and have retained it. (In this case, you don't update the revision, you just "undo" your plan changes.) You can "Request Review" again to get back to "Needs Review".

Then implements a "sticky accept" switch:

  - When off, updates downgrade accepts.
  - When off, "request review" always downgrades accepts.

Test Plan:
  - Went through all (I think?) of the plan changes / request review / accept / update workflows, with sticky accept on and off.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T3202

Differential Revision: https://secure.phabricator.com/D8614