Add of a partial template for messages

I added a partial for messages that will be prompt to user in case of
success, warning, failure, ...
Signed-off-by: Matthieu Berjon <matthieu.berjon@inria.fr>

django/templates/partials/_messages.html

+{% for message in messages %}
+  <div class="alert {{ message.tags }} alert-dismissible" role="alert">
+    <button type="button" class="close" data-dismiss="alert" aria-label="Close">
+      <span aria-hidden="true">&times;</span>
+    </button>
+    {{ message }}
+  </div>
+{% endfor %}