Allow master view to decide whether each grid checkbox is checked

aka. un-break what the v3 grids broke..

tailbone/views/master2.py

@@ -258,6 +258,7 @@ class MasterView2(MasterView):
             'url': lambda obj: self.get_action_url('view', obj),
             'checkboxes': self.checkboxes or (
                 self.mergeable and self.request.has_perm('{}.merge'.format(self.get_permission_prefix()))),
+            'checked': self.checked,
         }
         if 'main_actions' not in kwargs and 'more_actions' not in kwargs:
             main, more = self.get_grid_actions()