Firestore

[rwieruch]

Write the next version of the book completely with Firestore.

• remove or repurpose the Firestore Migration Tutorial: https://www.robinwieruch.de/react-firestore-tutorial/
• replace a few, but not all, "realtime-database" mentions with "cloud firestore"
• take code from https://github.com/the-road-to-react-with-firebase/react-firestore-authentication