upgrades: factor out Stripe integration.

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.

test/test_helpers/stripe_test_helper.rb

@@ -3,11 +3,11 @@ StripeMock.webhook_fixture_path = "test/fixtures/stripe-webhooks"
 module StripeTestHelper
   def mock_stripe!
     setup do
-      StripeMock.start unless UserUpgrade.enabled?
+      StripeMock.start unless PaymentTransaction::Stripe.enabled?
     end
 
     teardown do
-      StripeMock.stop unless UserUpgrade.enabled?
+      StripeMock.stop unless PaymentTransaction::Stripe.enabled?
     end
   end
 end