Save id_token for the SSO logout

As far as I can see, Keycloak ignores it entirely. I can login in as SSO
user A, then disconnect from the SSO directly and reconnect as user B,
but when I disconnect user A from AUR, Keycloak disconnects B even
though AUR passed it an ID token for A.

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>