Warning when deleting uploads or entries that are referenced

With all references (and schema uses) stored in the searchable metadata, it should be possible to implement a check and warning when deleting uploads and entries.

This might be a bit harder when a folder is deleted. But, using the text-based path search on mainfile even this should be feasible.

If we do a search on delete anyways, the delete confirm dialog could also provide information on how much files/data is about to be deleted.