Fix two transactional issues around revision status

Summary:
Two issues:

  - Herald is currently overwriting accepts and rejects with "blocking reviewer". Just stop it from doing that.
  - When you update an accepted revision, we put it back in "needs review", then return it to "accepted", generating an extra transaction. Instead, don't.

Test Plan:
  - Updated a revision with an accepting, herald-based blocking project reviewer. Reviewer was still accepting.
  - Updated an accepted revision, didn't get an extra transaction.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Differential Revision: https://secure.phabricator.com/D8506