After discovering a couple of bugs, I decided to thoroughly test
and rewrite this integration from scratch. The older code wasn't
generating coherent messages.
This also commit gets this integration up to 100% test coverage.
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.