user upgrades: add upgrade code system.

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.

app/logical/payment_transaction/shopify.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+class PaymentTransaction::Shopify < PaymentTransaction
+  class InvalidWebhookError < StandardError; end
+
+  def create!(**params)
+    nil
+  end
+
+  def refund!(reason = nil)
+    raise NotImplementedError
+  end
+
+  concerning :WebhookMethods do
+    class_methods do
+      def receive_webhook(request)
+        verify_webhook!(request)
+      end
+
+      private def verify_webhook!(request)
+        payload = request.body.read
+        actual_signature = request.headers["X-Shopify-Hmac-Sha256"].to_s
+        calculated_signature = Base64.strict_encode64(OpenSSL::HMAC.digest("sha256", Danbooru.config.shopify_webhook_secret, payload))
+        raise InvalidWebhookError unless ActiveSupport::SecurityUtils::secure_compare(actual_signature, calculated_signature)
+
+        request
+      end
+    end
+  end
+end