The fixture changes are because self.upgrade formerly used to cause a page load
of /billing, which in turn calls Customer.retrieve.
If we ran the full test suite with GENERATE_STRIPE_FIXTURES=True, we would
likely see several more Customer.retrieve.N.json's being deleted. But
keeping them there for now to keep the diff small.
When we started the billing system we started by following conventions used
in the Stripe documentation, but in hindsight it makes more sense to follow
conventions used in the Zulip codebase.
Also fixes a bug in process_initial_upgrade. If you have a card on file
(e.g. from a previous subscription), and try to upgrade by billing by
invoice, neither the if nor the elif condition applies.
The actual implementation of the change will be a cron job that runs once a
day and generates invoices for anyone with an account_balance > 0.
There are currently no tests for that part of the flow, so no tests had to
change.
