Contribute: Support upstream Open edX deployments (other than Tahoe)

[OmarIthawi]

Currently, Course Access Groups is tightly coupled with our edx-organizaiton fork due to the need for the UserOrganizationMapping model.

This is not set in stone and can be abstracted out to support vanilla Open edX deployments, but that would be new work. I created this issue to track the work.

What's the UserOrganizationMapping model?

It's part of our edX Platform customization to make Open edX more multi-tenant. We have this customization in our edx-organizations fork.

We initially made Course Access Groups with the intent to be used on Tahoe so we left few tight-coupling within. This shouldn't be the case since it's possible to support both modes of operations: Tahoe Multi-tenancy mode and Vanilla Open edX mode.