Webhooks send a POST to an admin-supplied URL when something happens,
with relevant information sent in JSON. One has been added for creating
users in Settings > Webhooks > User Created.
Lazily, the portion of GetUsers which generates a respUser has been
factored out, and is called to send the JSON payload.
A stripped-down common.Req method has been added, which is used by the
barebones WebhookSender struct.
