fix: messages coming from Django where never displayed in map view

umap/templates/umap/map_messages.html

@@ -1,10 +1,12 @@
 <script type="text/javascript">
+  window.addEventListener('DOMContentLoaded', (event) => {
     {% for m in messages %}
     {# We have just one, but we need to loop, as for messages API #}
-    L.U.fire('ui:alert', {
+    MAP.ui.alert({
       content: "{{ m }}",
       level: "{{ m.tags }}",
       duration: 100000
     })
   {% endfor %}
+})
 </script>