Convert to OAuth2 login provider instead of OIDC
This changes our use of `auth-backends` from the deprecated OIDC provider to the OAuth2 provider, because I rebuilt my devstack and the OIDC one wasn't working.
This is working, however there are a couple issues:
Superuser permission doesn't seem to be applied via SSO - when I login as a user who has staff and superuser permissions, the blockstore user only has staff permission
perhaps related to [this](https://github.com/edx/auth-backends/pull/62) but I think I had that fix?
The OAuth2 backend by default requests the `user_id` scope, and based on [these](https://github.com/edx/auth-backends/pull/59) [two](https://github.com/edx/edx-platform/pull/20057) PRs, that should work, but the default devstack LMS is giving me an `invalid_scope` error when `user_id` is included. Not sure why.
Test instructions: See the instructions added to the README.