Use a configurable responder when not signed in

The aim here is to to give Monban users full control over what happens
when `requires_login` is run and a `user` is not signed in.

* Default redirect location configurable with `requires_login_redirect`.
  This accepts anything that can be passed to `redirect_to`.
* A configuration for `requires_login_responder` was added. In the
  default configuration, this is a lambda that conforms to the interface
  of ActionController::Responder.

Using the responder interface seemed a good way to offer flexibility for
doing more advanced things (responding appropriately based on MIME
types, for instance).