* Changing the slug may also change the path if the slug duplicates another page, because the code that detects duplicate key errors and makes path and slug both unique can't distinguish the key responsible. So we must update the paths of our children as well if this happens. Unit test passes, also real world tests
* UI doesn't let you submit a request to set a page slug to one that would duplicate another page anymore when editing an existing page; you get a sensible alert instead. New pages and race conditions of editing page settings still benefit from the automatic de-duplication behavior