fix: rework backend admin to get it working again

[marius-mather]

Description

The backend admin that was originally set up stopped working, and in trying to fix it I discovered it had a few issues that meant it wasn't going to work long-term. starlette-admin is similar but copes better with our data (e.g. enums).

Changes

• Switch to starlette-admin for viewing backend data at /db-admin
• Add all existing models to it - no create/delete/edit permissions by default
• Use Auth0 for authentication - can only access admin with the correct role
• Use a separate Auth0 client ID/secret to log in to the admin - our existing backend app does not play well with it
  • The admin will be disabled if this is not configured
• Update unit tests of admin

Screenshots

[amandazhuyilan]

Looks great! Also tested on my machine that the backend admin dashboard can show up successfully on localhost:8000/db-admin. Only additional request is to add instructions on how to fire up the admin dashboard on the README 🙏 .

[amandazhuyilan]

Lets ship it (when all tests are green)