favgroups: make private favgroups a Gold-only option.

Make private favgroups a Gold-only option. This is for consistency with
private favorites and upvotes being Gold-only options.

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

app/policies/favorite_group_policy.rb

@@ -15,7 +15,11 @@ class FavoriteGroupPolicy < ApplicationPolicy
     update?
   end
 
+  def can_enable_privacy?
+    record.creator.is_gold?
+  end
+
   def permitted_attributes
-    [:name, :post_ids_string, :is_public, :post_ids, { post_ids: [] }]
+    [:name, :post_ids_string, :is_public, :is_private, :post_ids, { post_ids: [] }]
   end
 end