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.

else:
                arg_status_filter = "active" if hide_paused_dags_by_default else "all"
                flask_session[FILTER_STATUS_COOKIE] = arg_status_filter
        else:
            status = arg_status_filter.strip().lower()
            flask_session[FILTER_STATUS_COOKIE] = status
            arg_status_filter = status
        dags_per_page = PAGE_SIZE
        start = current_page * dags_per_page
        end = start + dags_per_page
        
        filter_dag_ids = get_airflow_app().appbuilder.sm.get_accessible_dag_ids(g.user)
        with create_session() as session:
            
            dags_query = session.query(DagModel).filter(~DagModel.is_subdag, DagModel.is_active)
            if arg_search_query:
                escaped_arg_search_query = arg_search_query.replace("_", r"\_")
                dags_query = dags_query.filter(
                    DagModel.dag_id.ilike("%" + escaped_arg_search_query + "%", escape="\\")
                    | DagModel.owners.ilike("%" + escaped_arg_search_query + "%", escape="\\")
                )
            if arg_tags_filter:
                dags_query = dags_query.filter(DagModel.tags.any(DagTag.name.in_(arg_tags_filter)))
            dags_query = dags_query.filter(DagModel.dag_id.in_(filter_dag_ids))
            filtered_dag_count = dags_query.count()
            if filtered_dag_count == 0 and len(arg_tags_filter):
                flash(
                    "No matching DAG tags found.",
                    "warning",
                )