Add groups fixture, and let it be loaded

I've put this fixture file in the xSACdb directory as it's not really
bound to one app, this needs to be added to settings.py to get included
in the search when doing ./manage.py loaddata
Implement role checking

Gives True/False functions, method decorators and CBV mixins
Implement context processors
enforce login on all pages, except /accounts/login/
fix login exempt urls to allow FB login
implement checking for instructor-ship
Implement permissions/roles/restrictions thingy across all current views
custom 403 page
Merge branch 'roles'