Implement redirect after SLO

[uprightech]

After SLO it is not possible in principle to redirect the user to a specific URL.
Implement this feature either using relayState or some other feature.
Care must be taken not to turn shibboleth into an open redirector (which is the reason the feature doesn't exist in the first place).

[uprightech]

There are migration issues from 4.5.1 to 4.5.5 which are being resolved.