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/