Resolves #641

- Add apartment gem
- Refactor seed script to work with multiple tenants
- Update College model to include subdomain
- Add custom middleware to handle invalid subdomains
- Modify the rack_canonical_host behavior to ignore the first subdomain
- Add an unauthenticated CollegesController#index action that only
  appears if a) the user is not signed in and b) there is no current
  college
- Disable JS tests because they break things :-(
- Add CollegeCreator to handle the creation of new tenants while also
granting a superuser access to said tenants
- Overhaul mailer views and add MailHelper to properly send e-mails to
different tenants
- Clean up bin/setup a bit