For unsupported or invalid payloads, we should just raise the
UnexpectedWebhookEventType exception and let our logging system
take care of recording the payload that caused the error.
This commit only adds support for the four events that have sample
payloads provided for them on the Pagerduty developer website.
Support for the remaining events will be added in subsequent
commits, as we get access to more sample payloads.
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.