Use a configurable responder when not signed in

Use a configurable handler 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 `no_login_redirect`.
  This accepts anything that can be passed to `redirect_to`.
* A configuration for `no_login_handler` was added. This allows the
  developer to easily completely customize the response to account for
  MIME types or whatever they please.

Ah ha "Make sure redirect ignores namespace" so yeah it needs to be there.