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.

config/danbooru_default_config.rb

@@ -363,6 +363,11 @@ module Danbooru
       resolution_mismatch source_larger source_smaller]
     end
 
+    # Whether the Gold account upgrade page should be enabled.
+    def user_upgrades_enabled?
+      true
+    end
+
     def stripe_secret_key
     end