Add support for all Firebase Emulators

[th0rgall]

https://firebase.google.com/docs/emulator-suite

This allows core contributors to more easily test and debug problems involving several Firebase components.

The goal is also that this should help provide a nearly-full development environment that can be publicly shared (excluding SendGrid & Stripe API tokens). SendGrid could be mocked.

We should attempt to make the emulators independent of a logged-in Firebase account or project.

• possibly by adding a demo- prefix project; https://stackoverflow.com/questions/67781589/how-to-setup-a-firebase-demo-project
• https://stackoverflow.com/questions/68726082/do-i-need-a-firebase-service-account-when-i-only-want-to-use-the-emulator