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

ab84ccdf3d5bb883f18d34133c30af410c7ff1d2.
Documentation for the new WebAPI errorss have been added to the manual.
NB: This work is just some minor fixups of work done by Steven MacLeod.

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.
Built and looked at the docs.