Create and send a "comment_removed" signal

[andymccurdy]

django.contrib.comments sends a comment_was_posted signal when a new comment is created. Marking a comment as removed (is_removed=True) should send a comment_removed signal for third party apps that need to take into account a comment being removed from the system.

[andymccurdy]

The same signal should also be triggered if a comment is deleted. So perhaps a post_delete on comments.get_model() could also send this signal.