Fix markup of modal close button in feedback template (#451)

Author: jgonggrijp

frontend/src/feedback/feedback-template.hbs

@@ -52,4 +52,4 @@
     </form>
 
 </div>
-<button type=button modal-close is-large" aria-label="close"></button>
+<button type=button class="modal-close is-large" aria-label="close"></button>