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/
Moves localized string to javascript, in the prompt to discard a draft review.

Making the last few changes on code that fixed an issue, where there was no
prompt after pressing the Discard Review button on draft reviews.

This moves embedded localized strings for the button text to use .val()
instead of concatenating them into HTML.

Testing Done:
Tested Visually (in both Chrome and Firefox)

Both tested with and without clicking 'ok' on the draft review under a comment or issue.

. Clicking the Discard Review button, then clicking cancel closes the prompt.
. Clicking the Discard Review button, then clicking discard exits the review and gets you back to the review request.

Bugs closed: 3300

Reviewed at https://reviews.reviewboard.org/r/6346/
Merge branch 'release-2.0.x'