Allow accepting accepted revisions, and rejecting rejected revisions

Summary:
Ref T1279. With the new per-reviewer status, you can always accept or reject a revision.

This is primarily cosmetic/UI changes. In particular, you've always been able to reject a rejected revision, the UI just didn't show you an option.

Test Plan: Accepted accepted revisions; rejected rejected revisions. See screenshots.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1279

Differential Revision: https://secure.phabricator.com/D7243