Spinner loading wheel on tree view

airflow/www/templates/airflow/tree.html

@@ -10,6 +10,8 @@
 {% block body %}
 {{ super() }}
 <div id="svg_container">
+  <img id='loading' width="50" 
+    src="{{ url_for('static', filename='loading.gif') }}">
   <svg class='tree' width="100%"></svg>
 </div>
 {% endblock %}
@@ -240,6 +242,7 @@ function update(source) {
     d.y0 = d.y;
   });
 
+  $('#loading').remove()
 }
 
 function set_tooltip(){