R1:eda64b85497b

Add a very basic EditPro controller for Differential

Summary: Ref T11114. This doesn't really support anything yet, but technically works if you manually go to `/editpro/`.

Test Plan: {F2117302}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17043