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.

Reviewers: btrahan, testuser1122344

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3718

Differential Revision: https://secure.phabricator.com/D6756