Delete a webhook endpoint

Changes apply to events from now on, and to deliveries still waiting for a retry: each is checked against the endpoint as it is when its turn comes. A delivery whose event the endpoint no longer takes (its new types or profiles) ends unsent, with `endpoint_changed` on its attempt; the ones it still takes go to its new `url`. A new `url` starts with a closed breaker, makes the caller (or the person behind the API key or agent) the person it sends for (so does any change to one that sends for nobody, `onBehalfOf` null), and the deliveries it had already tried or held at the old address end with `endpoint_changed` (replay them to send them to the new one). `enabled` false stops it, and its waiting deliveries end at once with `endpoint_disabled`; true starts it again for new events, also after a 410 Gone or days of failures, with its failures forgotten. What ended while it was off is not sent by itself: replay it (`Replay an endpoint's deliveries`).