Add closing/reopening signals

Review Request #7332 — Created May 25, 2015 and discarded — Latest diff uploaded


Review Board


Fire "review_request_closing" when attempting to close a review request.
If a signal receiver raises a CloseError abort the close operation.
This signal allows extensions to prevent a review request from being
closed. Alert the user in the UI when a close request has failed.

Also introduce a new "review_request_reopening" signal which behaves the
same way as the closing signal but deals with reopening review requests.

These new signals expand upon publishing signals that were previously
added to allow extensions control over publishing - see commit

Wrote a dummy extension to raise the new errors inside of the new
signals - API requests were properly blocked and the error messages
were alerted to the user.

Compared change description behaviour to Review Board without this
change and observed it to be unchanged after the patch.