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.

DagRun.data_interval_start,
            DagRun.data_interval_end,
        ).join(
            last_runs_subquery,
            and_(
                last_runs_subquery.c.dag_id == DagRun.dag_id,
                last_runs_subquery.c.max_execution_date == DagRun.execution_date,
            ),
        )
        resp = {
            r.dag_id.replace(".", "__dot__"): {
                "dag_id": r.dag_id,
                "state": r.state,
                "execution_date": wwwutils.datetime_to_string(r.execution_date),
                "start_date": wwwutils.datetime_to_string(r.start_date),
                "end_date": wwwutils.datetime_to_string(r.end_date),
                "data_interval_start": wwwutils.datetime_to_string(r.data_interval_start),
                "data_interval_end": wwwutils.datetime_to_string(r.data_interval_end),
            }
            for r in query
        }
        return flask.json.jsonify(resp)
    @expose("/code")
    @auth.has_access(
        [
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG_CODE),
        ]
    )
    def legacy_code(self):