Decide how deleted forms impact deleting datasets

[ktuite]

First brought up in

• Dataset attachments replaced with static CSVs still show up linked to forms #1626

Does the dataset metadata stuff check for deleted forms?

If you're able to delete a dataset after deleting a form, what does that mean for restoring the form? If you're not able to do so, what is it like to be stuck with a dataset you can't delete because a soft-deleted form still has a reference to it?