**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

airflow/www/views.py
]
    )
    @action_logging
    @provide_session
    def gantt(self, session=None):
        """Show GANTT chart."""
        dag_id = request.args.get('dag_id')
        dag = current_app.dag_bag.get_dag(dag_id)
        demo_mode = conf.getboolean('webserver', 'demo_mode')
        root = request.args.get('root')
        if root:
            dag = dag.sub_dag(task_ids_or_regex=root, include_upstream=True, include_downstream=False)
        dt_nr_dr_data = get_date_time_num_runs_dag_runs_form_data(request, session, dag)
        dttm = dt_nr_dr_data['dttm']
        form = DateTimeWithNumRunsWithDagRunsForm(data=dt_nr_dr_data)
        form.execution_date.choices = dt_nr_dr_data['dr_choices']
        tis = [ti for ti in dag.get_task_instances(dttm, dttm) if ti.start_date and ti.state]
        tis = sorted(tis, key=lambda ti: ti.start_date)
        ti_fails = list(
            itertools.chain(
                *[
                    (
                        session.query(TaskFail)
                        .filter(
                            TaskFail.dag_id == ti.dag_id,
                            TaskFail.task_id == ti.task_id,
                            TaskFail.execution_date == ti.execution_date,
                        )
                        .all()
                    )