Moved emitting user_registered signal to djblets, to prevent sending false emails.

Emitting this signal from the registration view wasn't correct, since that view
had no way of knowing whether or not registration was actually successful. Now
that djblets emits this signal instead, we can ensure that the new user
registration emails are only sent when a registration actually occurs.

Testing Done:
All unit tests pass.

Submitted blank, invalid, valid registration forms while:
1. no user was logged in.
2. another user was logged in

Bugs closed: 3438

Reviewed at https://reviews.reviewboard.org/r/6336/