Add a webhook endpoint

Adds an https address that receives the chosen events, signed in the Standard Webhooks format. The address must be on port 443 and resolve to public addresses only (400 otherwise). The answer carries the signing secret (whsec_...) once; it cannot be shown again. 429 at the limit or when adding too fast, 503 when the server cannot seal secrets. Needs an owner or editor; personal access tokens receive 403.

Required token scopes are listed in x-hootlens-scopes. A signed-in person needs the matching permission on the site instead (the viewer and editor roles are presets of the permissions).