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.

func.sum(case((DatasetEvent.id.is_not(None), 1), else_=0)).label("total_updates"),
                )
                .join(DatasetEvent, DatasetEvent.dataset_id == DatasetModel.id, isouter=not has_event_filters)
                .group_by(
                    DatasetModel.id,
                    DatasetModel.uri,
                )
                .order_by(*order_by)
            )
            if has_event_filters:
                count_query = count_query.join(DatasetEvent, DatasetEvent.dataset_id == DatasetModel.id)
            filters = [~DatasetModel.is_orphaned]
            if uri_pattern:
                filters.append(DatasetModel.uri.ilike(f"%{uri_pattern}%"))
            if updated_after:
                filters.append(DatasetEvent.timestamp >= updated_after)
            if updated_before:
                filters.append(DatasetEvent.timestamp <= updated_before)
            query = query.filter(*filters)
            count_query = count_query.filter(*filters)
            query = query.offset(offset).limit(limit)
            datasets = [dict(dataset) for dataset in query.all()]
            data = {"datasets": datasets, "total_entries": count_query.scalar()}
            return (
                htmlsafe_json_dumps(data, separators=(",", ":"), cls=utils_json.WebEncoder),
                {"Content-Type": "application/json; charset=utf-8"},
            )
    @expose("/robots.txt")
    @action_logging
    def robots(self):