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/views/user_upgrades/create.js.erb

@@ -10,6 +10,9 @@
     var $input = $('<input type="hidden" name="token">').val(token);
     $form.append($input).appendTo("body").submit();
   });
+<% elsif @user_upgrade.shopify? %>
+    window.history.pushState({}, "", location.href);
+    window.location.assign("<%= j Danbooru.config.shopify_purchase_url %>?attributes[user_upgrade_id]=<%= @user_upgrade.id %>&attributes[purchaser_id]=<%= @user_upgrade.purchaser_id %>&attributes[purchaser_name]=<%= @user_upgrade.purchaser.name %>&checkout[email]=<%= @user_upgrade.purchaser&.email_address&.address %>");
 <% else %>
   <% raise NotImplementedError, "payment method not implemented" %>
 <% end %>