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).
I'm not sure we need requires_login_redirect since that would be encompassed by requires_login_responder and I don't see much point in having a configuration option just to be used by another configuration option yet. That might change but for now I think just having the responder is fine. Additionally, it's possible this current responder won't issue a response so a name for it could be `unauthenticated_action_handler` which seems to imply and cover the ground this should be used for. Any thoughts on this?