message: Adding owner(s) to dashboard makes them own underlying slices
diff: @@ -453,6 +453,9 @@ class DashboardModelView(SupersetModelView, DeleteMixin): # noqa
  obj.owners.append(g.user)
  utils.validate_json(obj.json_metadata)
  utils.validate_json(obj.position_json)
  + owners = [o for o in obj.owners]
  + for slc in obj.slices:
  + slc.owners = list(set(owners) | set(slc.owners))
  def pre_update(self, obj):
  check_ownership(obj)
  