Add ApplicationTransactions to Herald

Summary: Ref T2769. I'm planning to keep this pretty simple, but we have this ad-hoc edit log for rules already and some other mess that we can clean up.

Test Plan: No effect yet; see future changes.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2769

Differential Revision: https://secure.phabricator.com/D6654