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.

session.query(TaskInstance.state, sqla.func.count(TaskInstance.dag_id))
            .filter(TaskInstance.dag_id == dag_id)
            .group_by(TaskInstance.state)
            .all()
        )
        active_runs = models.DagRun.find(dag_id=dag_id, state=DagRunState.RUNNING, external_trigger=False)
        tags = session.query(models.DagTag).filter(models.DagTag.dag_id == dag_id).all()
        
        owner_links = session.query(DagOwnerAttributes).filter_by(dag_id=dag_id).all()
        attrs_to_avoid = [
            "schedule_datasets",
            "schedule_dataset_references",
            "task_outlet_dataset_references",
            "NUM_DAGS_PER_DAGRUN_QUERY",
            "serialized_dag",
            "tags",
            "default_view",
            "relative_fileloc",
            "dag_id",
            "description",
            "max_active_runs",
            "max_active_tasks",
            "schedule_interval",
            "owners",
            "dag_owner_links",
            "is_paused",
        ]
        attrs_to_avoid.extend(wwwutils.get_attr_renderer().keys())
        dag_model_attrs: list[tuple[str, Any]] = [
            (attr_name, attr)