Simplify and improve PhabricatorCustomField APIs
Summary:
Ref T1703. Ref T3718. The PhabricatorCustomFieldList seems like a pretty good idea. Move more code into it to make it harder to get wrong.
Also the sequencing on old/new values for these transactions was a bit off; fix that up.
Test Plan: Edited standard and custom profile fields.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T1703, T3718
Differential Revision: https://secure.phabricator.com/D6751