Implement soft-deletes for datasets & labels

[ThisIsMissEm]

Currently deletes are a hard-delete, resulting in the dataset becoming a 404 Not Found, when it would be more appropriate to return a 410 Gone response status.

This would still be irreversible, but we'd keep the dataset identifier in the database, such that we can handle the 410 gone case. This will either be via a deletions table using a polymorphic approach, or via a deleted_at timestamp, haven't decided yet. Deleting a dataset will still delete all the changes stored within it, we just need to preserve the identifier such that we can return the correct status code.