[auth] Improve how we define admin accounts

[humphd]

In #1796, we are defining admin accounts via an env variable with ADMINISTRATORS being passed in. This is probably OK for one super-admin, but the rest can be defined in Firebase with the #1642 (cc @chrispinkney).

[humphd]

I don't think we're using the ADMINISTRATORS env stuff at all in the auth service. We do in the old backend, see

telescope/src/backend/data/admin.js

Line 6 in 29951b7

const administrators = process.env.ADMINISTRATORS ? process.env.ADMINISTRATORS.split(' ') : [];

.

Let's add it to the auth service as well, so we can bootstrap a few super-user admins into the system without the chicken-and-egg problem of database access.