Bring very basic EditEngine support to commits

Summary:
Ref T10978. After T11114, we have some features (like the old code for the haunted comment panel) which are only used by Diffusion. I want to modernize it so I can nuke them. T10978 also describes many bugs which are only fixable after modernizing.

This adds very basic EditEngine support for commits/audit. You can't create new commits with this workflow, just tag/update existing ones.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10978

Differential Revision: https://secure.phabricator.com/D17175