R1:b7c584137f40

Begin generalizing custom fields

Summary:
Ref T1703. We have currently have two custom field implementations (Maniphest, Differential) and are about to add a third (User, see D6122). I'd like to generalize custom fields before doing a third implementation, so we don't back ourselves into the ApplicationTransactions corner we have with Maniphest/Differential/Audit.

For the most part, the existing custom fields work well and can be directly generalized. There are three specific things I want to improve…