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.

)
                current_dags = current_dags.outerjoin(
                    dag_run_subquery, and_(dag_run_subquery.c.dag_id == DagModel.dag_id)
                )
                null_case = case((dag_run_subquery.c.max_execution_date.is_(None), 1), else_=0)
                if arg_sorting_direction == "desc":
                    current_dags = current_dags.order_by(
                        null_case, dag_run_subquery.c.max_execution_date.desc()
                    )
                else:
                    current_dags = current_dags.order_by(null_case, dag_run_subquery.c.max_execution_date)
            else:
                sort_column = DagModel.__table__.c.get(arg_sorting_key)
                if sort_column is not None:
                    null_case = case((sort_column.is_(None), 1), else_=0)
                    if arg_sorting_direction == "desc":
                        current_dags = current_dags.order_by(null_case, sort_column.desc())
                    else:
                        current_dags = current_dags.order_by(null_case, sort_column)
            dags = current_dags.options(joinedload(DagModel.tags)).offset(start).limit(dags_per_page).all()
            user_permissions = g.user.perms
            can_create_dag_run = (
                permissions.ACTION_CAN_CREATE,
                permissions.RESOURCE_DAG_RUN,
            ) in user_permissions
            dataset_triggered_dag_ids = {dag.dag_id for dag in dags if dag.schedule_interval == "Dataset"}
            if dataset_triggered_dag_ids:
                dataset_triggered_next_run_info = get_dataset_triggered_next_run_info(
                    dataset_triggered_dag_ids, session=session
                )