Add a system for upgrading accounts using upgrade codes. Users purchase
an upgrade code off-site then redeem it on-site to upgrade their account
to Gold. Upgrade codes are randomly pre-generated and are one time use
only. Codes have enough randomness that guessing a code is infeasible.
* 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.
