manager: wait for onload before running switch-user js (#42294)

src/authentic2/manager/templates/authentic2/manager/user_su.html

@@ -13,7 +13,7 @@
         <a id='su-link' href="{{ su_url }}">{{ su_url }}</a>
     </p>
     <script>
-      window.a2_js_seconds_until();
+      $(function() { window.a2_js_seconds_until(); });
     </script>
 </form>
 {% endblock %}