R1:7821c9805374

Use PhabricatorCustomField infrastructure to render Releeph custom fields on the edit screen

Summary:
Ref T3718. This moves custom field rendering on the edit screen to PhabricatorCustomField and makes all the APIs conformant.

We still run through edit with both old-school and new-school sets of fields, because the actual editing isn't on the new stuff yet. That will happen in a diff or two.

Test Plan: Edited a request; intentionally introduced errors and verified the form behaved as expected.