Remove hard dependency on `sites` framework

[dokterbob]

Django's sites framework is hardly used and, honestly, kind of sucks. AFAIK all builtin Django apps are now designed to work with but not depend on the sites framework. It would be great to do the same for django-newsletter, and preferably use the same mechanism which Django uses for this type of soft dependency.

[woodz-]

@dokterbob will #357 solve your issue?