Implement Accept/Reject in ApplicationTransactions, approximately

Summary: Ref T2222. This mostly makes Accept/Reject work. The big missing piece is that overall revision status does not yet update properly. I need to think about how I want that to work a little bit more.

Test Plan: Accepted and rejected some stuff.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8333