Enabled hook_user_logout functionality

Okay, I rerolled the patch without the form validation and made an issue fork and merge request.

I have two questions:

1. Should it log something when the user logs out? It never has, but it feels like this might be useful.

2. The end of the function looks like this:

  // Force redirect if saml_request is not empty.
  if(!empty($saml_request)) {
    $redirect = new RedirectResponse($url);
    $redirect->send();
  }

Is it possible for $saml_request to be empty? What would happen?

My IdP isn’t set up for single logout, so I can’t easily test that this actually works.