Restore the Differential "edit" and "reply" keyboard shortcuts

Summary: Ref T12616. This makes "edit" and "reply" work again.

Test Plan:
Used "e" and "r" to edit and reply.

Also used them in bogus ways and got useful UI feedback.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12616

Differential Revision: https://secure.phabricator.com/D17895