Add offline auth sources

## Meta info

- enhancement suggestion
- up-to-date master branch
- webapp running on own infrastructure

## Context
We are looking into using web-maker as the front end for delivering simple web-development lessons in a low-bandwidth school environment in Himalayan India (see the [skeletal website](https://code-at-the-edge.github.io/) and [github repos](https://github.com/code-at-the-edge/) for a little more information).
Students in the classes in general do not have email addresses and often do not have phone numbers, so the built-in firebase auth methods will not work for them. We will in any case be running the application server from a local node in the classroom with no access to the Internet, so remote authentication will be difficult or impossible.

## Request

It looks like firebase [supports creation of new users via the SDK](https://firebase.google.com/docs/auth/admin/manage-users#create_a_user), and I guess there is a [firebase admin node module](https://www.npmjs.com/package/firebase-admin). Is there any chance that you could bolt an admin interface for adding accounts onto the server? I imagine there are significant security model issues that we ourselves are probably not equipped to think through, though we will try to do something ourselves if we need to.