"Flatten" as an action - to remove a parent topic and automatically move children up to the current topic

Could be a useful feature when it comes to un-nesting topics?

[Original report](https://www.notion.so/learningequality/Flatten-as-an-action-to-remove-a-parent-topic-and-automatically-move-children-up-to-the-current-t-454f577d75b547458359c4f37b60ab2a)