settings: Replace user groups delete button with fa-trash-o.

This makes this settings page a bit more consistent with the rest of
the site.

static/templates/admin_user_group_list.hbs

@@ -10,10 +10,10 @@
             {{t 'Saved' }}
         </button>
         <button class="button save-status btn-danger small">
-            {{t 'Discard changes' }}
+            <i class="fa fa-undo" aria-label="{{t 'Delete' }}" title="{{t 'Delete' }}"></i>
         </button>
         <button class="button rounded small delete btn-danger">
-            {{t 'Delete' }}
+            <i class="fa fa-trash-o" aria-label="{{t 'Delete' }}" title="{{t 'Delete' }}"></i>
         </button>
     </h4>
     <p class="subscribers">{{t 'Subscribers' }}</p>