Make several actions ask again, in one request

The batch primitive behind a group control: removes the CALLING member's consent for every id in override_ids, resolving the caller's own actor ONCE. Idempotent per item exactly like the single-action route above — an id belonging to nobody or to another member reports revoked: false rather than an error, so this is never an existence oracle for other people's settings. Same preferences:read scope as the single-action route, for the same reason: revoking only ever takes authority away.