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.
Fixes flaky tests:
* The resolvers handle users in "bins", which are groups that depend on the user ID.
* The test user ID varies depending on the test order.
* This change ensures that the bin requested matches the user for the test.
00653433a5