Use CustomField, not AuxiliaryField, to power RevisionView

Summary: Ref T2222. This will probably have some rough edges for a bit (e.g., weird cases I didn't remember or think of), but there's no change to the underlying data and we can easily revert if things get too messy.

Test Plan: Looked at a variety of revisions and saw sensible output.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8361