Philo

Currently, tree entities always have a slug field, because they were originally built as an abstract subclass for nodes and templates, which have slug-based paths. Shipherd navigation items are also tree-based entites, but with no need for a slug - and this is probably not the only such case. Since trees do not imply paths, it would make sense to make a TreeEntity that subclasses Entity and MPTTModel and then a separate PathModel/PathEntity that add the path-based functionality that relies on the slug.