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/models/user_upgrade.rb

@@ -22,6 +22,8 @@ class UserUpgrade < ApplicationRecord
   enum payment_processor: {
     stripe: 0,
     authorize_net: 100,
+    shopify: 200,
+    upgrade_code: 300,
   }
 
   scope :gifted, -> { where("recipient_id != purchaser_id") }
@@ -113,7 +115,7 @@ class UserUpgrade < ApplicationRecord
   end
 
   concerning :UpgradeMethods do
-    def process_upgrade!(payment_status)
+    def process_upgrade!(payment_status = "paid")
       recipient.with_lock do
         return unless pending? || processing?
 
@@ -174,6 +176,8 @@ class UserUpgrade < ApplicationRecord
         PaymentTransaction::Stripe.new(self)
       in "authorize_net"
         PaymentTransaction::AuthorizeNet.new(self)
+      in "shopify"
+        PaymentTransaction::Shopify.new(self)
       end
     end