* Rename the stripe_id column to transaction_id.
* Add a new payment_processor column to identity the processor used for
this transaction (and hence, which backend system the transaction_id is for).
Factor out the Stripe code from the UserUpgrade class. Introduce a new
PaymentTransaction abstract class that represents a payment with some
payment processor, and a PaymentTransaction::Stripe class that
implements transactions with Stripe.
Note that we can't completely eliminate Stripe even though we no longer
accept payments with it because we still need to be able to look up old
payments in Stripe.