votes: make private favorites and upvotes a Gold-only option.

Make private favorites and upvotes a Gold-only account option.

Existing Members with private favorites enabled are allowed to keep it
enabled, as long as they don't disable it. If they disable it, then they
can't re-enable it again without upgrading to Gold first.

This is a Gold-only option to prevent uploaders from creating multiple
accounts to upvote their own posts. If private upvotes were allowed for
Members, then it would be too easy to use fake accounts and private
upvotes to upvote your own posts.

app/policies/user_policy.rb

@@ -31,6 +31,10 @@ class UserPolicy < ApplicationPolicy
     user.is_admin? || record.id == user.id || !record.enable_private_favorites?
   end
 
+  def can_enable_private_favorites?
+    user.is_gold?
+  end
+
   def permitted_attributes_for_create
     [:name, :password, :password_confirmation, { email_address_attributes: [:address] }]
   end