Webhooks: use hooks in memory when generating callback calls
Webhooks: block multiple subscriptions to the same callback URL

Will allow only one subscription to a given callback URL. If trying to
subscribe again to the same URL, will return success but with a message
saying the callback URL is duplicated, very similar to the response of
create when called multiple times.
Webhooks: sign callback calls with a checksum

Simple signature done initially in a way similar to how bbb-web does it.
Webhooks: fix callbacks being called multiple times