R1:026137f92fe6

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…