WIP: Fix "Subscription failed - no active Service Worker"

Fix Subscription failed - no active Service Worker by waiting for the Service Worker registration to be active before trying to subscribe registration.pushManager.subscribe()

• https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorkerRegistration/active

Enable push notifications locally

General docs on web push, https://developers.google.com/web/fundamentals/push-notifications/web-push-protocol

1. node ./scripts/utils/feature-toggle.js --name web-push --description "Enable web-push notifications" --enable-off
2. Visit http://localhost:5000/api_web/features/web-push/1 to enable web-push feature
3. ... todo ...

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Other references,

• https://developers.google.com/web/fundamentals/codelabs/push-notifications/
• https://developers.google.com/web/fundamentals/codelabs/debugging-service-workers/#testing_push_notifications
• https://developers.google.com/web/ilt/pwa/introduction-to-push-notifications
• https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorkerRegistration/showNotification
• https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorkerRegistration/getNotifications

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Closes https://gitlab.com/gitlab-org/gitter/webapp/issues/1892