The main remaining todo for correctly populating
RealmAuditLog.requires_billing_update is supporting the de-seating (and
corresponding re-seating) that happens after being offline for two weeks.
This completes the separation of our logic for managing Stripe
customers from the view code for the billing page.
As we add more features to our Customer model and to our Stripe
integration, we might further separate those two things; but for now
they're nearly synonymous and there's no problem in them being mixed
together.