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.

views.go

@@ -666,7 +666,7 @@ func (app *appContext) NewUserFromConfirmationKey(invite Invite, key string, lan
 		profile = &p
 	}
 
-	nu := app.NewUserPostVerification(NewUserParams{
+	nu /*wg*/, _ := app.NewUserPostVerification(NewUserParams{
 		Req:                 req,
 		SourceType:          sourceType,
 		Source:              source,
@@ -707,6 +707,9 @@ func (app *appContext) NewUserFromConfirmationKey(invite Invite, key string, lan
 	delete(invKeys, key)
 	app.ConfirmationKeys[invite.Code] = invKeys
 	app.confirmationKeysLock.Unlock()
+
+	// These don't need to complete anytime soon
+	// wg.Wait()
 	return
 }