[Future enhancement / issue tracking] Deleting units leaves behind children

As identified in this Slack thread, deleting units may cause unexpected deletion or unexpected preservation of child objects, and this issue will extend to sections and subsections. This issue is intended to document and track potential fixes, including adjusting structural block deletion behavior or adding a "cleanup" feature.

There are several product options when deleting a structural block:

1. Delete all children (and their children)
2. Preserve all children
3. Delete all exclusive children (children without any other parents) and repeat recursively
4. Delete all children authored from inside the structural block (and repeat recursively)
5. Preserve all children with other parents, and all children authored from outside the structural block (or its children). Delete all others.
6. By default, preserve all children, but allow the user to opt-in to deleting exclusive children.

For Teak, we are pursuing option 2 (preserve all children). After this is implemented, in user testing, we should watch out for some key user issues.

• As identified in the Slack thread linked above, users may be surprised when components they authored inside a unit are "left behind" when that unit is deleted
• Users who are used to authoring in courses may expect libraries to behave similarly, so we should evaluate whether this library paradigm is clear to them (and if not, whether it becomes more familiar over time or is a blocking issue)
• Users may be frustrated by left-behind components building up. If this is the case, we should evaluate whether they are satisfied or frustrated with manual deletion / search-and-filter workflows to avoid being interrupted by those floating components

Read the linked Slack thread and locate the existing structural block deletion behavior in the frontend-app-authoring codebase; no file, test, or entry point is named. The issue tracks potential fixes for Teak, with option 2 (preserve all children) under consideration, followed by user testing of the listed concerns.