Bug 1075998 - Use soft deletion for add-on reviews, and allow admins to ...

...restore them.
This looks scary and dangerous, and could cause many unforeseen side effects. In which cases does this fail mysteriously?

Having a special case for reviews, where you would not use this shortcut but the full version including `with_deleted` seems safer (I'm talking about the [Review.undelete](https://github.com/mozilla/olympia/pull/374/files#diff-9fa99c74cd309b7df98b4fcb71d5ef14R101) method).