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.

base_order = ("id", "created_date")
    formatters_columns = {
        "created_date": wwwutils.datetime_f("created_date"),
    }
class TaskInstanceModelView(AirflowPrivilegeVerifierModelView):
    """View to show records from TaskInstance table."""
    route_base = "/taskinstance"
    datamodel = AirflowModelView.CustomSQLAInterface(models.TaskInstance)  
    class_permission_name = permissions.RESOURCE_TASK_INSTANCE
    method_permission_name = {
        "list": "read",
        "action_clear": "edit",
        "action_muldelete": "delete",
        "action_set_running": "edit",
        "action_set_failed": "edit",
        "action_set_success": "edit",
        "action_set_retry": "edit",
        "action_set_skipped": "edit",
    }
    base_permissions = [
        permissions.ACTION_CAN_CREATE,
        permissions.ACTION_CAN_READ,
        permissions.ACTION_CAN_EDIT,
        permissions.ACTION_CAN_DELETE,
        permissions.ACTION_CAN_ACCESS_MENU,
    ]
    page_size = PAGE_SIZE
    list_columns = [
        "state",
        "dag_id",