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