Provide comment actions for tokenizer custom fields

Summary:
Ref T13114. See PHI519. An install is interested in modifying a tokenizer custom field from the comment area. Provide this capability.

This patch is fairly narrow but should solve the immediate need.

Test Plan: Added, removed, and modified a tokenizer custom field using the comment action dropdown.

Maniphest Tasks: T13114

Differential Revision: https://secure.phabricator.com/D19270