3.1 - Implement Data Layer

[BeatriceMitchell]

We need to have a Firebase data layer that starts working immediately after the user updates the Firebase credentials. It will be mocked until those are changed.

Child Tasks: #45, #35

[BeatriceMitchell]

Note: Need to reach out to Andrew to clarify whether the Firebase can be pretend or needs to be easily hooked up.

[BeatriceMitchell]

To quote from the Proposal:

The module should include a mock data layer and a Firebase-backed data layer.
The mock data layer should be enabled by default.
In other words, the code on GitHub should have a blank value for Firebase credentials. If unmodified, the code detects the blank value and automatically uses the mock data layer.

The Firebase data layer should be ready to work out of the box whenever the developer adds in their credentials, and the README/instructions should reflect that.

[BeatriceMitchell]

@Jan-Stepien Create two tasks: One for mocking the data layer, and one for the Firebase-backed data layer

[BeatriceMitchell]

Per discussion with Google, there is no need for actual mocking. If they have credentials, it initializes the Firebase repo and uploads to Firestore. If they don't have any credentials, it doesn't communicate at all.

[BeatriceMitchell]

Current time estimate is about 3 days