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.

app/controllers/webhooks_controller.rb

@@ -8,7 +8,7 @@ class WebhooksController < ApplicationController
   def receive
     case params[:source]
     when "stripe"
-      UserUpgrade.receive_webhook(request)
+      PaymentTransaction::Stripe.receive_webhook(request)
       head 200
     when "discord"
       json = DiscordSlashCommand.receive_webhook(request)