**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
response = self.client.get(
            "{}?dag_id={}&task_id={}&execution_date={}&link_name=foo-bar".format(
                self.endpoint, self.dag.dag_id, self.task.task_id, self.default_date
            ),
            follow_redirects=True,
        )
        self.assertEqual(response.status_code, 200)
        response_str = response.data
        if isinstance(response.data, bytes):
            response_str = response_str.decode()
        self.assertEqual(
            json.loads(response_str),
            {
                'url': 'http://www.example.com/some_dummy_task/foo-bar/2017-01-01T00:00:00+00:00',
                'error': None,
            },
        )
    def test_global_extra_links_works(self):