The DAGs main page has All/Active/Paused.

It would be nice if we can have also Running Now which will show all the DAGs that have at least 1 in Running of DAG runs:
![Screen Shot 2021-08-25 at 11 30 55](https://user-images.githubusercontent.com/62940116/130756490-647dc664-7528-4eb7-b4d8-515577f7e6bb.png)


For the above example the new DAGs tab of Running Now should show only 2 rows.
The use case is to see easily what DAGs are currently in progress.

[
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_TASK_INSTANCE),
        ]
    )
    @action_logging
    @provide_session
    def task(self, session: Session = NEW_SESSION):
        """Retrieve task."""
        dag_id = request.args.get("dag_id")
        task_id = request.args.get("task_id")
        execution_date = request.args.get("execution_date")
        dttm = _safe_parse_datetime(execution_date)
        map_index = request.args.get("map_index", -1, type=int)
        form = DateTimeForm(data={"execution_date": dttm})
        root = request.args.get("root", "")
        dag = get_airflow_app().dag_bag.get_dag(dag_id)
        if not dag or task_id not in dag.task_ids:
            flash(f"Task [{dag_id}.{task_id}] doesn't seem to exist at the moment", "error")
            return redirect(url_for("Airflow.index"))
        task = copy.copy(dag.get_task(task_id))
        task.resolve_template_files()
        ti: TaskInstance | None = (
            session.query(TaskInstance)
            .options(
                
                
                
                joinedload(TaskInstance.queued_by_job, innerjoin=False),
                joinedload(TaskInstance.trigger, innerjoin=False),