Add Wagtail CMS for kfet app.

K-Fêt
- Integrate wagtail to serve "static" pages of old K-Fêt website
- Fixture "kfetcms/kfet_wagtail_17_05" contains a copy of old website
(as in May 2017).
- Media files can be got until end of June 17 at
http://partage.eleves.ens.fr//files/604e6dea2ceebc66b1936c6b3f911744/kfet_media.tar.gz

Login/logout
- Update package django_cas_ng to last version.
- Clean COFCASBackend.
- Change CAS version to 3 (version used on eleves.ens). This enables
the logout redirection (for CAS ofc).
- Add messages and clean existing ones on login/logout (for both
outsider and cas users).

Misc
- Update settings to bypass an incompability between debug-toolbar and
wagtailmenus packages.
- Better management of dev/test-specific urls (if debug-toolbar wasn't in
INSTALLED_APPS, media files were not served).
- UI improvements.

gestioncof/templates/home.html

@@ -1,4 +1,5 @@
 {% extends "gestioncof/base_header.html" %}
+{% load wagtailcore_tags %}
 
 {% block homelink %}
 {% endblock %}
@@ -55,7 +56,8 @@
             <h3 class="block-title">K-Fêt<span class="pull-right"><i class="fa fa-coffee"></i></span></h3>
             <div class="hm-block">
                 <ul>
-                    <li><a href="{% url "kfet.home" %}">Page d'accueil</a></li>
+                    {# TODO: Since Django 1.9, we can store result with "as", allowing proper value management (if None) #}
+                    <li><a href="{% slugurl "k-fet" %}">Page d'accueil</a></li>
                     <li><a href="https://www.cof.ens.fr/k-fet/calendrier">Calendrier</a></li>
                     {% if perms.kfet.is_team %}
                     <li><a href="{% url 'kfet.kpsul' %}">K-Psul</a></li>