Feat/OIDC logout

[joeftiger]

Provides a new environment variable OIDC_LOGOUT_URL to correctly log out users from their OpenID Connect provider.

This fixes #213.

I am not entirely convinced yet about directly providing client_id and redirect_uri as uri parameters, but this could be a sensible default value. Maybe someone more knowledgeable about the OIDC protocol can chip in :smiley: .