Add a webhook

An https address on the public internet and the events to send it. The answer carries the signing secret, this once. Each delivery is a POST of { id, type, created_at, data } with Pagefully-Signature: t=<unix seconds>,v1=<hex>, where v1 is HMAC-SHA256 with the secret over the timestamp, a full stop and the body as sent. Refuse one more than 300 seconds old, and dedupe by id: a delivery is made at least once. Anything but a 2xx is tried again, 6 attempts in all, after 60, 300, 1800, 7200, 43200 seconds. No redirect is followed. An address on a private or local network is refused. A workspace holds 5. There is no page.live and no page.rejected. Pagefully makes a page in App Store Connect and does not submit it for review, and it does not read Apple's verdict back, so it cannot say that a page went live or was rejected.