**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
"""Refresh DAG."""
        dag_id = request.values.get('dag_id')
        orm_dag = session.query(DagModel).filter(DagModel.dag_id == dag_id).first()
        if orm_dag:
            orm_dag.last_expired = timezone.utcnow()
            session.merge(orm_dag)
        session.commit()
        dag = current_app.dag_bag.get_dag(dag_id)
        
        current_app.appbuilder.sm.sync_perm_for_dag(dag_id, dag.access_control)
        flash(f"DAG [{dag_id}] is now fresh as a daisy")
        return redirect(request.referrer)
    @expose('/refresh_all', methods=['POST'])
    @auth.has_access(
        [
            (permissions.ACTION_CAN_EDIT, permissions.RESOURCE_DAG),
        ]
    )
    @action_logging
    def refresh_all(self):