Remove sites framework from site

[cproctor]

I originally had an idea of supporting multitenancy (e.g. private installations for specific schools) via the sites framework. However, this ends up feeling clumsy. If I do support multiple installations in the future, I'll probably use django-tenants. For now, the sites framework just adds cruft to the codebase. Need to remove:

• Sites foreign keys from models
• Usages of sites-based managers in views

And test.