Refactor register/update logic from action_plugin_authsaml::handle_login to saml_handler::login

[pv2b]

Fixed cases where automatic registration / user updates don't happen when user is logged in through force_saml_login, by cleaning up code so registration/updates always happen on login always happens no matter how the login was reached.

Also found and fixed a bug where the $this->users cache in action_plugin_authsaml was incorrectly updated.