Add basic support for push notifications via Firebase

[hsjoberg]

Add basic support for receiving push normal visible notifications.
No silent or no wake-up to sync the chain. Let's just start with the bare minimum first.

Later on we can build upon this to wake up up the app to sync and even to receive payments via Lightning Box (~dependent on lightningnetwork/lnd#6427 and if that works out well).

Maybe use https://github.com/wix/react-native-notifications as it's made by the well-established Wix company.