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.

test/factories/favorite_group.rb

@@ -2,5 +2,10 @@ FactoryBot.define do
   factory :favorite_group do
     creator
     name { SecureRandom.uuid }
+
+    factory :private_favorite_group do
+      creator factory: :gold_user
+      is_public { false }
+    end
   end
 end