ComponentTreeItem should automatically purge subtrees for slots that are no longer exposed

With 📌 [PP-1] Consider not storing the ComponentTreeStructure data type as a JSON blob Postponed , that would become a lot simpler to achieve.

Just seeing this now. I believe this requires some consideration regarding how users would move exposed slots from one place to another within a template. The fact that slots were getting purged later enabled users to achieve this without a specific UI affordance.

In the meanwhile, we could think through how we'd implement this with publish workflow. If you delete a slot, that change should only take place during publish, not when the slot gets deleted from the template.