Add a confirmation when discarding a pending review from the review dialog.

When a user has a draft review, and opens up the "Edit Review" box to finalize
their comments and publish, the "Discard Review" command did not confirm at
all. Now it gives a prompt to discard or cancel.

Testing Done:
Tested visually, by publishing a review; both pressing and not pressing 'ok'
under the comment.

Case 1: Clicking Discard Review, then clicking cancel. It takes away the prompt.
Case 2: Clicking Discard Review, then clicking discard. Discards the review and
goes back to the review request.

Bugs closed: 3300

Reviewed at https://reviews.reviewboard.org/r/6341/