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.

logmessages/logmessages.go

@@ -307,6 +307,9 @@ const (
 	CaptchaNotFound       = "Captcha \"%s\" not found in invite \"%s\""
 	FailedVerifyReCAPTCHA = "Failed to verify reCAPTCHA: %v"
 	InvalidHostname       = "invalid hostname (wanted \"%s\", got \"%s\")"
+
+	// webhooks.go
+	WebhookRequest = "Webhook request send to \"%s\" (%d): %v"
 )
 
 const (