Drop specifying redis all the time, but allow not calling webhooks
    
    The redis argument that was provided was not used in its original purpose
    which was to only send notifications to webhook if redis was properly
    configured, but it's now all handled by the workers and celery.
    However, in one place, the default hook, it was use to allow by-passing
    sending webhook notifications.
    So to keep this behavior in place, we introduced a dedicated argument.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>