Add Declarative Web Push

This introduces a new feature whereby push messages conforming to a certain JSON format directly create an end user notification and show it (possibly preceded by an enhanced push event).

In addition to showing a notification, the app badge can be updated as well.

This builds on whatwg/notifications#213 which adds URL members to notifications.

Exposing PushManager outside of service workers is handled by #393.

Author: annevk