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.

script/fixes/110_generate_upgrade_codes.rb

@@ -0,0 +1,8 @@
+#!/usr/bin/env ruby
+
+require_relative "base"
+
+5_000.times do
+  code = UpgradeCode.create!(creator: User.system)
+  puts "id=#{code.id} code=#{code.code}"
+end