Move registration_form to gestioncof/

The `registration` view was referencing this file using
`"gestioncof/registration_form.html"` whereas the `registration_post`
templated was referencing it in the global namespace
`"registration_form.html"`.

We choose the `gestioncof/` folder in order to avoid further conflicts
with other apps.

gestioncof/templates/gestioncof/registration_post.html

@@ -3,6 +3,6 @@
 {% block realcontent %}
         <h2>Inscription d'un nouveau membre</h2>
         <div id="form-placeholder">
-        {% include "registration_form.html" %}
+        {% include "gestioncof/registration_form.html" %}
         </div>
 {% endblock %}