webhooks: add "user created" webhook

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.

models.go

@@ -1,6 +1,8 @@
 package main
 
-import "time"
+import (
+	"time"
+)
 
 type stringResponse struct {
 	Response string `json:"response" example:"message"`