This kind of payload that's loaded from json in the body of the request
is not only used for webhooks, but also in the push bouncer, and may get
used elsewhere too - so a general name is better.
This converts most webhook integration views to use @typed_endpoint instead
of @has_request_variables, rewriting REQ parameters. For these
webhooks, it simply requires switching the decorator, rewriting the
type annotation of payload/message to WebhookPayload[WildValue], and
removing the REQ default that defines the to_wild_value converter.
