We need to achieve only one process for all booking, not one for subscription, one for reservations, etc.
Moreover we must store one object per invoice_item/payment_schedule_object and stop using Invoice.invoiced or PaymentSchedule.scheduled
- from stripe only to payment gateway abstract
- from reservation params style to unified cart_items
TODO: fix not passing tests,
subscriptions/renew_as_admin_test.rb fails because InvoiceItem(9).amount = nil