Fix/602: root folder deletion

[T4g1]

Summary

• Fix [Bug] Unable to delete root folder #602
  When removing root folders that are contained within other root folders, audiobooks can be detected as using both of them.
  I fixed that issue by making it count the amount of root folders associated with each audiobooks and allowing the deletion only if none of them is orphaned from all root folders

Testing

• Reworked the root folders tests and added two tests to prevent regression and validate the behavior

Notes

• Some other areas of the root folder services assumes root folders are not contained within each other, we might want to create an issue to investigate the behavior regarding those and decide if we want to prevent that use case or not