Given an an activation key, look up and activate the user
account corresponding to that key (if possible).
+ The return value should be a 2-tuple (new-user, extra_context).
+ ``None`` can be given for the user if activation failed.
After successful activation, the signal
``registration.signals.user_activated`` will be sent, with the
newly activated ``User`` as the keyword argument ``user`` and
def registration_allowed(self, request):