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.

form=form,
                is_dag_run_conf_overrides_params=is_dag_run_conf_overrides_params,
                recent_confs=recent_confs,
            )
        try:
            execution_date = timezone.parse(request_execution_date)
        except ParserError:
            flash("Invalid execution date", "error")
            form = DateTimeForm(data={"execution_date": timezone.utcnow().isoformat()})
            return self.render_template(
                "airflow/trigger.html",
                form_fields=form_fields,
                dag_id=dag_id,
                origin=origin,
                conf=request_conf,
                form=form,
                is_dag_run_conf_overrides_params=is_dag_run_conf_overrides_params,
                recent_confs=recent_confs,
            )
        dr = DagRun.find_duplicate(dag_id=dag_id, run_id=run_id, execution_date=execution_date)
        if dr:
            if dr.run_id == run_id:
                message = f"The run ID {run_id} already exists"
            else:
                message = f"The logical date {execution_date} already exists"
            flash(message, "error")
            return redirect(origin)
        
        if run_id and "/" in run_id:
            flash(