WIP: Resolve "Roles"

The role system defined here is very simple and done at a "view" level. In order to perform it, the requiring of a third library called Django-braces is used and replaces the mixin.py file that was hosting the all GroupRequired mixin that was buggy and not generic enough. Roles are defined in Django as "group" and checked on a per-view basis. The roles are described in the wiki. I removed the inria group which isn't statisfying to create webapps (following #243 (closed)).

Closes #190