Feature Request: Delete files on disk when the corresponding object is deleted

[berrnd]

Currently products, recipes, equipment, users and userfields (of type file or image) can reference files (stored in data/storage, a separate subfolder for products, recipes and so on).

Those files get currently only deleted when the user does this by e.g. deleting a product picture on the product edit page, not automatically when deleting the whole product - the same goes for recipes and all other entities.

So somehow auto-delete the corresponding files on disk when such an object is deleted. And also delete once all currently unreferenced files (create a corresponding migration job).

References #2425.