no refs
- Updates member model serializer to directly set subscriptions on member object instead of `stripe.subscriptions`
- Updates all references to members subscriptions from nested `stripe.subscriptions` to `subscriptions`
- Updates v3 API serializer to still use `stripe.subscriptions`
- Updates tests
- Add a query param that indicates whether signin/up succeeded or failed
- Add unit tests for all 3 possible cases for the createSessionFromMagicLink middleware
- Added an acceptance test to show the behaviour works in principle
