createUnsubscribeToken(userId: string,cadence: DigestCadence): Promise<string>
Signs the token of a digest's unsubscribe link, which turns off cadence
for userId without signing in. It never expires, so the link in an old
email still works; turning the digest back on doesn't revoke it, which is
harmless since all it can do is turn it off again.
cadence: DigestCadence