code: def add_relationship_panels(self):
          
          from wagtailplus.utils.edit_handlers import add_panel_to_edit_handler
          from wagtailplus.wagtailrelations.edit_handlers import RelatedPanel
  
          for model in self.applicable_models:
              add_panel_to_edit_handler(model, RelatedPanel, _(u'Related'))
docs: Add edit handler that includes "related" panels to applicable
          model classes that don't explicitly define their own edit handler.