Implement symfony firewall logout success_handler.

[richgerdes]

In the Symfony docs, its recommended to implement logout events by specifying a logout.success_handler for the firewall. This project should supply a handler for use in applications.

In the Symfony Security documentation it recommends the following.

Need more control of what happens after logout? Add a success_handler key under logout and point it to a service id of a class that implements LogoutSuccessHandlerInterface.