Ability to see sub-tasks in the planning board, and move these into the sprint

We use sub-tasks to break down our work into implementable chunks. It would be useful to be able to drag these sub-tasks into the sprint from the planning board. The alternative is to use something like ALM structure, but it has more complexity than we need. Just being able to see sub-tasks in the backlog (possibly indented under the parent), and drag them into a sprint would be ideal.

Without this capability, we are not able to plan sprints appropriately.