Switch to Firebase Admin SDK for push notifications

One-sentence description

Why?

Possible deprecation of a used package.

Current implementation

We're currently using pyFCM which is using an older version of the Firebase Cloud Messaging APIs. There is a newer API (FCM v1) which has not been implemented.

Suggested implementation

Use the Firebase Admin SDK to communicate with the Firebase servers and send messages. https://firebase.google.com/docs/admin/setup

This package is also open source and is maintained by Firebase/Google instead of being a community project.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information