**Description**

Airflow DAG Run UI (Browse -> DAG Runs) is really useful for managing many DAG Runs, creating specific DAG Runs and so on.
It would be great to have an additional option to "With Selected DAG Runs" -> "Clear" to reset all task instances under those DAG Runs.

**Use case / motivation**

When rerunning DAGs especially during development cycles it is tedious to go into the Tree view and clear many DAG Runs manually. It would be great to have some way to do this in bulk / over a range of DAG Runs. 

The DAG Run UI seems like a good place to add this since it is already a kind of administrative area over all the DAG Runs and has "with selected" options like "delete", "mark failed", etc. Discussion welcome :)

**Related Issues**
None

airflow/www/views.py
end_date = failed_task_instance.end_date or timezone.utcnow()
            start_date = failed_task_instance.start_date or end_date
            if tf_count != 0 and failed_task_instance.task_id == prev_task_id:
                try_count = try_count + 1
            else:
                try_count = 1
            prev_task_id = failed_task_instance.task_id
            gantt_bar_items.append((failed_task_instance.task_id, start_date, end_date, State.FAILED,
                                    try_count))
            tf_count = tf_count + 1
            task = dag.get_task(failed_task_instance.task_id)
            task_dict = alchemy_to_dict(failed_task_instance)
            task_dict['state'] = State.FAILED
            task_dict['operator'] = task.task_type
            task_dict['try_number'] = try_count
            task_dict['extraLinks'] = task.extra_links
            tasks.append(task_dict)
        data = {
            'taskNames': [ti.task_id for ti in tis],
            'tasks': tasks,
            'height': len(tis) * 25 + 25,
        }
        session.commit()
        return self.render_template(
            'airflow/gantt.html',
            dag=dag,
            execution_date=dttm.isoformat(),
            form=form,
            data=data,
            base_date='',