This commit adds a test for the payload that is generated when
a Task is moved from one user story to another on Taiga's Sprint
Taskboard UI.
This commit also gets up this webhook's test coverage up to 100%.
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.