Guarantee the order of groups in the dropdown

So the groups with the same parent are listed together.

The recursive cte seemed to do it by itself, but it is not
guaranteed. By ordering on the `route.path` that includes every parent
group, we can assume they're going to be in the right order.

app/helpers/namespaces_helper.rb

@@ -4,8 +4,11 @@ module NamespacesHelper
   end
 
   def namespaces_options(selected = :current_user, display_path: false, extra_group: nil)
-    groups  = current_user.manageable_groups.includes(:route)
-    users   = [current_user.namespace]
+    groups = current_user.manageable_groups
+               .joins(:route)
+               .includes(:route)
+               .order('routes.path')
+    users = [current_user.namespace]
 
     unless extra_group.nil? || extra_group.is_a?(Group)
       extra_group = Group.find(extra_group) if Namespace.find(extra_group).kind == 'group'