Make ManiphestCustomField actually implement the interface it ostensibly exposes

Summary: There's a bunch of stuff that lives only in AuxiliaryField which is called on objects which may be ManiphestCustomFields right now. This is basically a list of remaining API methods which need to be moved to the new stuff. This enables construction of new-style custom fields.

Test Plan: Created a sophisticated Maniphest custom field.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7013