Remove willWriteRevision/didWriteRevision hooks

Summary:
Ref T2222. DifferentialRevisionEditor has no remaining callsites, but it has a bit of functionality which still needs to be ported forward. I'm going to rip it apart piece by piece.

This removes the willWriteRevision/didWriteRevision hooks. They are completely encapsulated by transactions now, except for a unique piece of branch/task logic, which I migrated forward.

Test Plan:

• Lots of grep.
• Created a new revision on branch T25, saw it associate with the task.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8454