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 this feature, and modern field infrastructure has moved away from selectors.
Test Plan: Viewed and edited projects, branches, and requests in Releeph. Grepped for removed config. Grepped for field_selector.
Reviewers: btrahan
Reviewed By: btrahan
CC: LegNeato, aran
Maniphest Tasks: T3661, T3718
Differential Revision: https://secure.phabricator.com/D6750