New codelab: Firebase for the frontend

We could add a new codelabs using Firebase and the Cloud Firestore intance: instead of reloading the page to see if the thumbnail has been created, we could have Firebase get notified automatically when the thumbnail has been processed and update the UI automatically.

See issue #5 for the `processed` flag.

Start by reading the existing codelab structure and issue #5 for how the `processed` flag is used. Trace the current thumbnail-status flow, then define the Firebase and Cloud Firestore steps needed for automatic UI updates. Done means the codelab documents notification-driven updates without requiring a page reload.

Written by the indexing model from the issue text.