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.

kfet/cms/context_processors.py

@@ -0,0 +1,20 @@
+from kfet.models import Article
+
+
+def get_articles(request):
+    articles = (
+        Article.objects
+        .filter(is_sold=True, hidden=False)
+        .select_related('category')
+        .order_by('category__name', 'name')
+    )
+    pressions, others = [], []
+    for article in articles:
+        if article.category.name == 'Pression':
+            pressions.append(article)
+        else:
+            others.append(article)
+    return {
+        'pressions': pressions,
+        'articles': others,
+    }