Implement notification/push api

[raffis]

Is your feature request related to a problem? Please describe

No.

Describe the solution you'd like

I'd like to see notifications sent by the web notification api.

1. Implement service worker
2. Ask for notification permission
3. Register browser for push notification on the balloon server (See Web push notifications balloon#330)
4. (User must disable it by uncheck the web notification adapter Possibility to enable/disable notification types/adapters #180)

See server push implementation: gyselroth/balloon#330
Web push/notification howto: https://developers.google.com/web/ilt/pwa/introduction-to-push-notifications

This is a follow up issue for pwa's #82.

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Additional context

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