This changes the point-of-logout to be within the initial part of the
SAML logout flow, as per 5.3.2 of the SAML spec, processing step 2.
This also improves the logout redirect handling to use the global
redirect suggestion so that auto-login handling is properly taken into
account.
Added tests to cover.
Manual testing performed against keycloak.
For #4713
Extracted logout to the login service so the logic can be shared instead
of re-implemented at each stage. For this, the SocialAuthService was
split so the driver management is in its own class, so it can be used
elsewhere without use (or circular dependencies) of the
SocialAuthService.
During review of #4467
