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.

"airflow/grid.html",
            root=root,
            dag=dag,
            doc_md=doc_md,
            num_runs=num_runs,
            show_external_log_redirect=task_log_reader.supports_external_link,
            external_log_name=external_log_name,
            dag_model=dag_model,
            auto_refresh_interval=conf.getint("webserver", "auto_refresh_interval"),
            default_dag_run_display_number=default_dag_run_display_number,
            default_wrap=conf.getboolean("webserver", "default_wrap"),
            filters_drop_down_values=htmlsafe_json_dumps(
                {
                    "taskStates": [state.value for state in TaskInstanceState],
                    "dagStates": [state.value for state in State.dag_states],
                    "runTypes": [run_type.value for run_type in DagRunType],
                    "numRuns": num_runs_options,
                }
            ),
        )
    @expose("/calendar")
    @auth.has_access(
        [
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_TASK_INSTANCE),
        ]
    )
    @gzipped
    @action_logging
    def legacy_calendar(self):