Also adds `SWITCH_PLAN_TIER_AT_PLAN_END` for `CustomerPlan`
which will be used to mark status of remote server legacy
plans which are scheduled for an upgrade.
So that all child classes of BillingSession generate the same data
structure for customers that are created in Stripe, revise
`get_data_for_stripe_customer` to return a specific dataclass:
StripeCustomerData.
So that `update_or_create_stripe_customer` can work for Customer
objects with either a realm or remote_server, we create an abstract
base class, BillingSession, and implement a child class for the
current implementation of Customer objects with a realm.
Refactoring `update_or_create_stripe_customer` also moves
`create_stripe_customer` and `replace_payment_method` to the
BillingSession class.
Anders Kaseorg
Aman Agrawal
Tim Abbott
Lauryn Menard