- Adds the `unlinkable` action to the XBlock object sent to the frontend - Updates the `top_level_parent_key` reference when unlinking containers. If you unlink a Section with Subsections and Units, this updates the `top_level_parent_key` for the Subsections to `None` (they are the top level now), and the `top_level_parent_key` for the Units to the corresponding parent Subsection.