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/views/favorite_groups/new.html.erb

@@ -2,12 +2,7 @@
   <div id="a-new">
     <h1>New Favorite Group</h1>
 
-    <%= edit_form_for(@favorite_group) do |f| %>
-      <%= f.input :name, as: :string, required: true %>
-      <%= f.input :post_ids_string, label: "Posts", as: :text %>
-      <%= f.input :is_public, label: "Public" %>
-      <%= f.submit "Submit" %>
-    <% end %>
+    <%= render "form" %>
   </div>
 </div>