R1:74de24909bfb

Partially move Releeph custom fields to PhabricatorCustomField

Summary:
Fixes T3661. Ref T3718. This makes Releeph custom fields extend PhabricatorCustomField so we can start moving over other pieces of infrastructure (rendering, storage, etc) to run through the same pathways. It's roughly the minimum amount of work required to be able to move forward.

NOTE: This removes per-project custom field selectors. Fields are now configured for an entire install. My understanding is that Facebook does not use…