Add support for Firebase notifications

[klochowicz]

Add support for sending push notifications to 10101 app:

some links to get started:

• https://stackoverflow.com/questions/72749963/push-notifications-for-flutter-chat-app-without-firebase
• https://medium.com/@yshean/implementing-push-notifications-in-flutter-apps-aef98451e8f1

The PoC could include some initial notification, e.g. "Thanks for being a beta tester <3" or something simple.
Once the mechanism works we can discuss which notifications to hook up.

[klochowicz]

I've scheduled it into the iteration, as a few 1.3.0 tasks (and experiments) hinge upon this work to be completed.