Phone authentication mechanism

[justinklchan]

It would be really great if we could have phone authentication.

The barrier though is pricing:
https://firebase.google.com/pricing/?gclid=CjwKCAjw-YT1BRAFEiwAd2WRtqv5Ps1opIYfvqQkJPPTdr7v5JxUZrIGcY3bmn75SzRISrGD0GNf7RoCb_oQAvD_BwE

However, given that there is a real potential for adoption, individual states could potentially be able to fund cloud services like this.

The ask in this issue is to be able to add in the Firebase auth for development testing and have a flag to turn it on or off.

So if we decide we want it, we can turn it on, and add in the appropriate API key.

And if we decide a particular deployment doesn't need it. Then we can leave it off.

A good place to have this is during the onboarding sequence between the "Get started" page and the "Permissions" page.

The developers for this can share the API key privately for testing.