Add django-authlib to enable SSO using GitHub into Django admin

[sarayourfriend]

Problem

As part of #383, we use a GitHub group in Cloudflare Access to allow some folks through to the Django admin site. However, we then also need to create separate Django accounts for them, including temporary passwords, and such.

It would be nice if we had some kind of SSO (in addition to another option that doesn't rely on a third party) for logging into Django Admin, and GitHub is a good option because we already manage user ACL through GitHub teams anyway.

Description

Add https://github.com/matthiask/django-authlib to the Django API, and incorporate it into Django Admin. Only allow members of the teams @WordPress/openverse-maintainers and @WordPress/openverse-content-moderators to sign in with GitHub SSO. Assign an appropriate Django role for each team. For members of openverse-maintainers, assign them as admins. For openverse-content-moderators, assign them the content moderator role.

Alternatives

Manage Django admin users by hand.

Host our own OIDC provider and don't use GitHub's authentication at all, not even for Cloudflare Access.

Additional context

This is just a "nice to have", nothing more.