**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
exec_date = dates.days_ago(2)
        start_date = datetime(2020, 4, 10, 2, 0, 0)
        end_date = exec_date + timedelta(seconds=30)
        with create_session() as session:
            for task in self.dag.tasks:
                ti = TaskInstance(task=task, execution_date=exec_date, state="success")
                ti.start_date = start_date
                ti.end_date = end_date
                session.add(ti)
        url = f'gantt?dag_id={self.dag.dag_id}&execution_date={exec_date}'
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_in_response('"extraLinks":', resp)
        extra_links_grps = re.search(r'extraLinks\": \[(\".*?\")\]', resp.get_data(as_text=True))
        extra_links = extra_links_grps.group(0)
        self.assertIn('airflow', extra_links)
        self.assertIn('github', extra_links)
    def test_operator_extra_link_override_global_extra_link(self):
        response = self.client.get(
            "{}?dag_id={}&task_id={}&execution_date={}&link_name=airflow".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': 'https://airflow.apache.org', 'error': None})
    def test_extra_links_error_raised(self):