* 'contentType' should be one of: page, book, chapter, bookshelf.
* 'contentId' should be the relevant ID of that item type you'd like to handle permissions for.
* Providing an empty `role_permissions` array will remove any existing configured role permissions,
* so you may want to fetch existing permissions beforehand if just adding/removing a single item.
* You should completely omit the `owner_id`, `role_permissions` and/or the `fallback_permissions` properties
* 'contentType' should be one of: page, book, chapter, bookshelf.
* 'contentId' should be the relevant ID of that item type you'd like to handle permissions for.
* Providing an empty `role_permissions` array will remove any existing configured role permissions,
* so you may want to fetch existing permissions beforehand if just adding/removing a single item.
* You should completely omit the `owner_id`, `role_permissions` and/or the `fallback_permissions` properties