Added signal hook for deleted review requests.
We use signal hooks for building an upgrade compatible database representation of the state of Review Board that we are interested in. This involves us tracking when a reply/review/request is created and so forth. There becomes a disconnect between the internal representation on Review Board and our external store when a review request gets deleted.
Thus, we internally added tracking for when a review request gets deleted (often by a Review Board admin).

Wired up an extension to test this use case.
We are currently running this with an extension on a staging server.
Ran the unit tests.