Make RATS use the state field of flask-oidc to store information
    
    This allows to send information to the oidc provider that will return
    it once the authentication succeeded.
    This way, we do not have to store it on the session scope where it would
    be overridden if the user makes two requests concurrently and we do not
    have to store it in a database either.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>