upgrades: add authorize.net integration.

Add integration for accepting payments with Authorize.net.

https://developer.authorize.net/hello_world.html

app/controllers/webhooks_controller.rb

@@ -1,7 +1,8 @@
 # frozen_string_literal: true
 
 class WebhooksController < ApplicationController
-  skip_forgery_protection only: :receive
+  skip_forgery_protection only: [:receive, :authorize_net]
+
   rescue_with Stripe::SignatureVerificationError, status: 400
   rescue_with DiscordSlashCommand::WebhookVerificationError, status: 401
 
@@ -17,4 +18,9 @@ class WebhooksController < ApplicationController
       head 400
     end
   end
+
+  def authorize_net
+    PaymentTransaction::AuthorizeNet.receive_webhook(request)
+    head 200
+  end
 end