Significant increase in query speed for Part list

- Custom list method
- Cache PartCategory objects in memory

InvenTree/part/serializers.py

@@ -101,6 +101,8 @@ class PartSerializer(InvenTreeModelSerializer):
 
         return queryset.prefetch_related(
             'category',
+            'category__parts',
+            'category__parent',
             'stock_items',
             'bom_items',
             'builds',