Support tokenizer custom fields in bulk editor

Summary:
Ref T13025. This allows custom tokenizer fields, like a "Owning Group" field, to be edited with the bulk editor.

See PHI173 for some context.

Test Plan: Edited a custom "Owner" field (a project tokenizer) with the bulk editor.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13025

Differential Revision: https://secure.phabricator.com/D18877