A base module which allows your site to send push notifications.

It does not send push notifications, but can be used as a dependency in
other modules which provide this functionality.

This module does:
- Provide a service worker for web push
- Allows users to enable push notifications
- Stores those subscriptions
- Allows the user to manage his/her subscriptions
- TBD

It is currently in an early development phase, so expect code to break regularly.