**Description**

For a DAG that looks like this:

```
A >> B
```

If A fails, B goes into "upstream_failed" state. If a user then marks A "success", B will remain in "upstream_failed" state. It will not automatically start running. This scenario often happens if the failure of A is dealt with outside of Airflow and the user does not want Airflow to run A again. But he usually expect Airflow to run B after A is marked "success".

**Use case / motivation**

After A is marked "success", its downstream tasks B should be cleared automatically and get ready to be scheduled. To avoid changing this behaviour completely, there can be a toggle button next to "Success" that lets the user decide if he wants Airflow to automatically clear downstream tasks that are in "upstream_failed" state.
airflow/www/views.py
"""Mark task as failed."""
        dag_id = request.form.get('dag_id')
        task_id = request.form.get('task_id')
        origin = get_safe_url(request.form.get('origin'))
        execution_date = request.form.get('execution_date')
        confirmed = request.form.get('confirmed') == "true"
        upstream = request.form.get('failed_upstream') == "true"
        downstream = request.form.get('failed_downstream') == "true"
        future = request.form.get('failed_future') == "true"
        past = request.form.get('failed_past') == "true"
        return self._mark_task_instance_state(
            dag_id,
            task_id,
            origin,
            execution_date,
            confirmed,
            upstream,
            downstream,
            future,
            past,
            State.FAILED,
        )
    @expose('/success', methods=['POST'])
    @auth.has_access(