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.

class XComModelView(AirflowModelView):
    """View to show records from XCom table."""
    route_base = "/xcom"
    list_title = "List XComs"
    datamodel = AirflowModelView.CustomSQLAInterface(XCom)
    class_permission_name = permissions.RESOURCE_XCOM
    method_permission_name = {
        "list": "read",
        "delete": "delete",
        "action_muldelete": "delete",
    }
    base_permissions = [
        permissions.ACTION_CAN_CREATE,
        permissions.ACTION_CAN_READ,
        permissions.ACTION_CAN_DELETE,
        permissions.ACTION_CAN_ACCESS_MENU,
    ]
    search_columns = ["key", "value", "timestamp", "dag_id", "task_id", "run_id", "execution_date"]
    list_columns = ["key", "value", "timestamp", "dag_id", "task_id", "run_id", "map_index", "execution_date"]
    base_order = ("dag_run_id", "desc")
    base_filters = [["dag_id", DagFilter, lambda: []]]
    formatters_columns = {
        "task_id": wwwutils.task_instance_link,
        "timestamp": wwwutils.datetime_f("timestamp"),
        "dag_id": wwwutils.dag_link,
        "map_index": wwwutils.format_map_index,
        "execution_date": wwwutils.datetime_f("execution_date"),
    }
    @action("muldelete", "Delete", "Are you sure you want to delete selected records?", single=False)
    def action_muldelete(self, items):