Add support for mod_auth_* authentication modules at /users/extlogin.
If session authenticated via SSO::Apache expired, just visit /users/extlogin again.
Stop infinite redirection loop to the login_url if we are already there.

This can happen when authorize_login_delegation is true but
/users/extlogin is not configured in Apache to authenticate.
Fall back to the /users/login method.

Addressing
undefined method `allowed_to?' for nil:NilClass

This can happen when authorize_login_delegation is true but
/users/extlogin is not configured in Apache to authenticate.
Upon logout, land the user on simple page and offer /users/extlogin logon.