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/8047d84e1e12b95d83439d335e8ab4e7/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.