This PR removes multiple unneeded fields from the `HookTask` struct and adds the two headers `X-Hub-Signature` and `X-Hub-Signature-256`.
## ⚠️ BREAKING ⚠️
* The `Secret` field is no longer passed as part of the payload.
* "Breaking" change (or fix?): The webhook history shows the real called url and not the url registered in the webhook (`deliver.go`@129).
Close #16115
Fixes #7788
Fixes #11755
Co-authored-by: zeripath <art27@cantab.net>
