Added OAuth2 Permissions Scope Handler

This scope offers a simple claim, administrator, that is true iff the user is a superuser.

@rocha @dsjen @mulby
`is_staff` grants users access to the admin portal. `is_superuser` grants users actual permissions to view/change items in the admin portal. Now that I think about it, `is_staff` might be the better variable to use here. We already have that level of access. Granting us superuser access would inherently make LMS less secure.