add: self registration for new users

Using django-registration users can now self-register. Current implementation
requires username and email address, this should probably be changed to email
only.

Make sure the entry in DB models ``sites`` is properly set and an email backend
is configured. For developing ``django.core.mail.backends.console.EmailBackend``
is used, refer to https://docs.djangoproject.com/en/dev/topics/email/ for
further backends.