Fix “Submitting” state of Close issue input

If .btn-default isn’t removed, .btn-default and .btn-warning styles
conflict

public/js/app.js

@@ -643,7 +643,7 @@ function initIssue() {
 
             var $button = $(this);
 
-            $button.removeClass("btn-success");
+            $button.removeClass("btn-success btn-default");
             $button.addClass("btn-warning");
 
             $button.text("Submiting...");