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.

test/factories/upgrade_code.rb

@@ -0,0 +1,5 @@
+FactoryBot.define do
+  factory(:upgrade_code) do
+    creator factory: :user
+  end
+end