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.

updater.go

@@ -130,11 +130,6 @@ type Updater struct {
 	binary                                     string
 }
 
-// SetTransport sets the http.Transport to use for requests. Can be used to set a proxy.
-func (ud *Updater) SetTransport(t *http.Transport) {
-	ud.httpClient.Transport = t
-}
-
 func newUpdater(buildroneURL, namespace, repo, version, commit, buildType string) *Updater {
 	// fmt.Printf(`Updater intializing with "%s", "%s", "%s", "%s", "%s", "%s"\n`, buildroneURL, namespace, repo, version, commit, buildType)
 	bType := off