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.

children_states = {item["state"] for item in child_instances}
            group_state = next((state for state in wwwutils.priority if state in children_states), None)
            group_start_date = min(filter(None, children_start_dates), default=None)
            group_end_date = max(filter(None, children_end_dates), default=None)
            return {
                "task_id": task_group.group_id,
                "run_id": dag_run.run_id,
                "state": group_state,
                "start_date": group_start_date,
                "end_date": group_end_date,
            }
        def get_mapped_group_summaries():
            mapped_ti_query = (
                session.query(
                    TaskInstance.task_id, TaskInstance.state, TaskInstance.run_id, TaskInstance.map_index
                )
                .filter(
                    TaskInstance.dag_id == dag.dag_id,
                    TaskInstance.task_id.in_(child["id"] for child in children),
                    TaskInstance.run_id.in_(r.run_id for r in dag_runs),
                )
                .order_by(TaskInstance.task_id, TaskInstance.run_id)
            )
            
            mapped_tis: Mapping[str, Mapping[int, list[TaskInstance]]] = collections.defaultdict(
                lambda: collections.defaultdict(list),
            )
            for ti in mapped_ti_query:
                mapped_tis[ti.run_id][ti.map_index].append(ti)
            def get_mapped_group_summary(run_id: str, mapped_instances: Mapping[int, list[TaskInstance]]):