Dequire homepage

In addition to removing the login requirement on the homepage, a previous story broke some things on the visual side. Fixed it and added some tests.
Because of [`setup_session_user`](https://github.com/18F/C2/blob/dff7a95b7f8fa5d9e6fe4aefc02eae04b5022805/app/controllers/home_controller.rb#L29-L31), this would be better as `if session[:user].present?`.