get_active_sites() to return QuerySet

[OmarIthawi]

• added tests
• added order_by parameter
• refactored _get_active_tiers_uuids to isolate the Tiers import

This PR addresses an indirect bug in which Figures expects a QuerySet:

• https://sentry.io/organizations/appsembler/issues/2198232186/?project=145493&query=is%3Aunresolved

do_update_enrollment_data = False if date_for < today else True
    sites = get_sites()
    sites_count = sites.count()
    ^

TypeError: count() takes exactly one argument (0 given)