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/controllers/favorite_groups_controller.rb

@@ -19,7 +19,7 @@ class FavoriteGroupsController < ApplicationController
   end
 
   def new
-    @favorite_group = authorize FavoriteGroup.new
+    @favorite_group = authorize FavoriteGroup.new(creator: CurrentUser.user)
     respond_with(@favorite_group)
   end