API endpoint for stopping the execution of tasks

```python
        assignment = None
        for item in current_assignments():
            for task in item["tasks"]:
                if task["id"] == task_id:
                    assignment = item
                    break
        else:
            logger.info("Cannot cancel task %s: not found", task_id)
            request.setResponseCode(NO_CONTENT)
            request.finish()
            return NOT_DONE_YET
```