modqueue: clear detailed rejection form before each use.

Bug: in the modqueue, if a user does a detailed rejection of one post,
then tries to do a detailed rejection of another post, the form will
still be filled in with the rejection message from the previous post.

app/javascript/src/javascripts/mod_queue.js

@@ -36,6 +36,7 @@ ModQueue.initialize_detailed_rejection_links = function() {
 
 ModQueue.detailed_rejection_dialog = function() {
   $("#post_disapproval_post_id").val($(this).data("post-id"));
+  $("#detailed-rejection-dialog").find("form")[0].reset();
 
   $("#detailed-rejection-dialog").dialog({
     width: 500,