users: disable upgrading to platinum.

Disable the ability to upgrade to Platinum. The Platinum level still
exists, but users can no longer upgrade to it. Sales of Platinum are
being disabled in preparation of increasing the tag limit for Gold
users.

app/models/favorite_group.rb

@@ -61,7 +61,7 @@ class FavoriteGroup < ApplicationRecord
   def creator_can_create_favorite_groups
     if creator.favorite_groups.count >= creator.favorite_group_limit
       error = "You can only keep up to #{creator.favorite_group_limit} favorite groups."
-      if !creator.is_platinum?
+      if !creator.is_gold?
         error += " Upgrade your account to create more."
       end
       errors.add(:base, error)