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/management/commands/loadkfetdevdata.py

@@ -147,3 +147,9 @@ class Command(MyBaseCommand):
         # ---
 
         call_command('createopes', '100', '7', '--transfers=20')
+
+        # ---
+        # Wagtail CMS
+        # ---
+
+        call_command('kfet_loadwagtail')