Re-enable has_parent optimization

Because we have foreign_key for parent_id now, we are sure
that parent_id can't be nil anymore so we can check parent_id
directly.

app/models/namespace.rb

@@ -277,7 +277,7 @@ class Namespace < ApplicationRecord
   end
 
   def has_parent?
-    parent.present?
+    parent_id.present? || parent.present?
   end
 
   def root_ancestor