In cases where the webhook payload doesn't have the username for the
author of a particular commit (this can happen if the author doesn't
have a GitHub account or the author's email is not associated with
their GitHub account), we now use the author's full name to format
messages.
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.