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