Fix json string escape in tree view

[houqp]

close #8523.

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[kaxil]

Can we add a test for this? There are some good examples for twst_views.py for TriggerDag and checking response

Example:

airflow/tests/www/test_views.py

Lines 2225 to 2235 in 5a47e80

	@mock.patch('airflow.www.views.dagbag.get_dag')
	def test_trigger_endpoint_uses_existing_dagbag(self, mock_get_dag):
	"""
	Test that Trigger Endpoint uses the DagBag already created in views.py
	instead of creating a new one.
	"""
	mock_get_dag.return_value = DAG(dag_id='example_bash_operator')
	url = 'trigger?dag_id=example_bash_operator'
	resp = self.client.post(url, data={}, follow_redirects=True)
	mock_get_dag.assert_called_once_with('example_bash_operator')
	self.check_content_in_response('example_bash_operator', resp)

[houqp]

@kaxil test added.

[kaxil]

Thanks @houqp