Implement PhabricatorApplicationTransactionInterface in Differential

Summary:
Ref T4810. Ultimate goal is to let Harbormaster post a "build passed/failed" transaction. To prepare for that, implement `PhabricatorApplicationTransactionInterface` in Differential.

To allow Harbormaster to take action on //diffs// but have the transactions apply to //revisions//, I added a new method so that objects can redirect transactions to some other object.

Test Plan:
  - Subscribed/unsubscribed/attached/detached from Differential, saw transactions appear properly.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4810

Differential Revision: https://secure.phabricator.com/D8802