5.1

• Allow child form class to be overridden in the formsets Meta property of ClusterForm (Helder Correia)
• Add prefetch_related support to ParentalManyToManyField (Andy Chosak)
• Implement copy_child_relation and copy_all_child_relations methods on ClusterableModel (Karl Hobley)
• Fix: Fix behavior of ParentalKeys and prefetch_related() supplied with a lookup queryset (Juha Yrjölä)