Add user to project when locking themselves out

Automatically add the current user with their access level to the
project's user list if after updating it they can no longer access it
(i.e. by making it private).

Reimplements e08fc01af521334465e4cd356e8a55c8386960f7, but at the API
level (ie. the logic is now in project_update() function instead of
manage_proj_update.php), so Mantis behave consistently from GUI as well
as SOAP.

Fixes 0016554