Feature: Edit previously sent generic webhook message (support "PATCH" request type)

Discord webhooks have [a neat feature](https://docs.discord.com/developers/resources/webhook#edit-webhook-message) where you can edit a previously sent webhook later on by sending a "PATCH" request type.

Basically, after you do an initial "POST" webhook, the server will return an `id` field. You can then use this id when sending a PATCH to update the previously sent message.

Start by tracing the existing generic webhook flow from its initial POST and identify where the returned id is handled. Then determine how a PATCH request can use that id to update the previously sent message. Done means a previously sent generic webhook message can be edited through the documented Discord webhook behavior, with the existing flow still working.

Written by the indexing model from the issue text.