Restore "Close" and "Reopen" actions to Differential on EditEngine

Summary:
Ref T11114. This restores these actions as selectable in the comment area.

This does not implement one special rule ("Closing a revision in response to a commit is OK from any status.") but I have a note about that separately.

Test Plan: Closed and reopened revisions.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17108