Flesh out ApplicationTransactions/CustomField integration

Summary:
None of this code is reachable yet. See discussion in D6147. Ref T1703.

Provide tighter integration between ApplicationTransactions and CustomField. Basically, I'm just trying to get all the shared stuff into the base implementation.

Test Plan: Code not reachable.

Reviewers: chad, seporaitis

Reviewed By: chad

CC: aran

Maniphest Tasks: T1703

Differential Revision: https://secure.phabricator.com/D6149