Papertrail sends requests with the content type
`application/x-www-form-urlencoded`, with the payload parameter holding the
JSON body. This commit fixes the papertrail integration to use the payload
parameter in the request's POST data instead of trying to parse the
request's entire body as JSON.
Papertrail documentation here:
https://help.papertrailapp.com/kb/how-it-works/web-hooks#encoding
All webhook fixtures in zerver/fixtures/<webhook_name> have now
been moved to dedicated webhook-specific directories under
zerver/webhooks/<webhook_name>/fixtures, where <webhook_name> is
the name of the webhook.
Anders Kaseorg
Puneeth Chaganti
Eeshan Garg