Bug fix for ensuring location and category names are unique for common parent (#4361)

* Update Meta class for StockLocation and PartCategory

* Migration files

* Add extra unique requirements to  InvenTreeTree model

- unique_together does not work as expected with null values

InvenTree/stock/models.py

@@ -47,6 +47,12 @@ class StockLocation(InvenTreeBarcodeMixin, MetadataMixin, InvenTreeTree):
     Stock locations can be hierarchical as required
     """
 
+    class Meta:
+        """Metaclass defines extra model properties"""
+
+        verbose_name = _('Stock Location')
+        verbose_name_plural = _('Stock Locations')
+
     def delete_recursive(self, *args, **kwargs):
         """This function handles the recursive deletion of sub-locations depending on kwargs contents"""
         delete_stock_items = kwargs.get('delete_stock_items', False)