Add setting to disallow anonymous reviews

There's been the ``OSCAR_ALLOW_ANON_REVIEWS`` setting for it since version 0.1 of Oscar, but never any code to support it. There's quite a few templates that have a link to leave a review that should be updated.

First notified in https://github.com/tangentlabs/django-oscar/issues/984. I've removed it from 0.6 for now: https://github.com/tangentlabs/django-oscar/commit/7075ba659409cee5f76c3a611d211344c86b46f0