Scheduled emails show "unsubscribe" link if waffle switch `schedules.course_update_show_unsubscribe` is enabled, and settings.ACE_ENABLED_POLICIES respects `bulk_email_optout`. API endpoint allows GET/POST requests, which: * GET asks for confirmation of opt-out * POST accepts "unsubscribe" or "cancel", where "unsubscribe" creates the Optout entry, and "cancel" does nothing.