Bug 1014432 - Correctly track closing the create user modal

The analytics event for closing the create user modal was triggered
by calling self.modal.close(). close() could be called by the user via
addEventListener or when the user account had been successfully created.
This led to an inflation in the number of times that users had cancelled
creating a new user.

It was also possible that closing the create user modal wasn't being
tracked by Angular apps because they use their own modal system.