CASC-220: Support front channel SLO logout

According to new front channel SLO in the CAS server 4.0, I'm proposing this change.

The expected behaviour for the CAS client is to receive both a *SAMLRequest* parameter and a *RelayState* parameter, to invalidate the current session and redirect back to the CAS server using the *RelayState* parameter.

From my own experience, I know that a lot of customizations of the CAS server logout are done by calling specific logout requests of the CAS client applications (embedded in iframes or hidden images). Moreover, we talked with @serac to send logout request in parallel. So I think it makes sense to add the behaviour for the CAS client to receive a logout request without a *RelayState* parameter and avoid doing the redirection back to the CAS server.
I'm looking forward to your feedbacks especially on this.

I added quite a lof of new tests to ensure the expected behaviour.