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.