Web push notifications for pipeline status transition

We should send push notifications for pipeline status transitions to explicitly subscribed users. We should do this using webpush on the BE and PushManager on a ServiceWorker on the FE.

This will mean users won't have to keep a pipeline page tab open in order to smart poll for status updates to get notifications. When the status changes, sidekiq will execute a worker job that sends a push notification to all subscribed users.

POC: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15961 (very POC, will need some understanding of rails and js to get working)

https://developers.google.com/web/fundamentals/engage-and-retain/push-notifications/images/push-flow.gif

Web Push Notifications - Timely, Relevant, and Precise

Edited by Luke Bennett