feat: Implement team members endpoint for admin console

[rodmgwgu]

Description

Closes #227

Implemented the /users endpoint to be used in the team members screen on the Admin Console for the AuthZ for Course Authoring project.

Also found and fixed one issue: In the enforcer, the is_admin_or_superuser_check which applies the special case for staff and super user Django users (always are allowed), was not taking into account Org glob scopes.

Deadline: Verawood

How to test

Using a local dev tutor installation, call the following endpoint with required credentials:

http://local.openedx.io:8000/api/authz/v1/users

The user you use to test this should have permission to at least view one course or library.

You should see a response similar to this:

{
    "count": 2,
    "next": null,
    "previous": null,
    "results": [
        {
            "username": "admin",
            "full_name": "",
            "email": "admin@example.com",
            "assignation_count": 1
        },
        {
            "username": "contributor",
            "full_name": "Juan Contrubuidor",
            "email": "contributor@example.com",
            "assignation_count": 4
        }
    ]
}

You can also test with params, for example:

http://local.openedx.io:8000/api/authz/v1/users?orgs=OpenedX,WGU&scopes=course-v1%3AOpenedX%2BDemoX%2BDemoCourse,lib%3AWGU%3ACSPROB&search=cont&sort_by=email&order=desc

Please note: the scopes should be url-encoded.

Merge checklist:

Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

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