Further simplify PhabricatorCustomFieldInterface
Summary:
Ref T1703. Ref T3718. This introduces `PhabricatorCustomFieldAttachment`, which is just a fancy `array()`. The goal here is to simplify `PhabricatorCustomFieldInterface` as much as possible.
In particular, it can now use common infrastructure (`assertAttached()`) and is more difficult to get wrong.
Test Plan: Edited custom fields on profile.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T1703, T3718
Differential Revision: https://secure.phabricator.com/D6752