**Description**
In the trigger page, there is a text area to enter (optional) conf json. It would be great if a sample JSON can be provided programatically while defining DAG

**Use case / motivation**

This will improve usability of the UI that triggers a DAG

tests/www/test_views.py
super().tearDown()
        with create_session() as session:
            session.query(RTIF).delete()
    def test_rendered_template_view(self):
        """
        Test that the Rendered View contains the values from RenderedTaskInstanceFields
        """
        self.assertEqual(self.task1.bash_command, '{{ task_instance_key_str }}')
        ti = TaskInstance(self.task1, self.default_date)
        with create_session() as session:
            session.add(RTIF(ti))
        url = 'rendered-templates?task_id=task1&dag_id=testdag&execution_date={}'.format(
            self.percent_encode(self.default_date)
        )
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response("testdag__task1__20200301", resp)
    def test_rendered_template_view_for_unexecuted_tis(self):
        """
        Test that the Rendered View is able to show rendered values
        even for TIs that have not yet executed
        """
        self.assertEqual(self.task1.bash_command, '{{ task_instance_key_str }}')
        url = 'rendered-templates?task_id=task1&dag_id=task1&execution_date={}'.format(
            self.percent_encode(self.default_date)
        )
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response("testdag__task1__20200301", resp)
    def test_user_defined_filter_and_macros_raise_error(self):