Add confirmation when discarding a review request.

This prompts the user for confirmation when discarding a review request through
Close -> Discarded, which will help prevent accidental discards.

Testing Done:
On a review request, I clicked Close -> Discarded, and got the confirmation
dialog. Clicking "Cancel" left the review request unchanged, and clicking "OK"
discarded the review request.

Bugs closed: 3117

Reviewed at https://reviews.reviewboard.org/r/5659/