Reference site and information for push notification

Need to implement push application server and encryption for client data to push

• https://developers.google.com/web/updates/2016/03/web-push-encryption Highly recommended to use library for encryption
• https://github.com/web-push-libs/web-push

Latest browser can handle VAPID doesn't need GCM key, but if we want to cover some old browser need old GCM method especially, samsung browser need to use GCM key for currently status