Add a tombstone for deleted datasets

[jrdh]

When a dataset is deleted by the owner the DOI resolves to a 404 page, for example: https://doi.org/10.5519/vnsz2n4q. It would be much better to show a page informing the user that the dataset has been removed and possibly other information? What information we could show requires investigation.

[KellyStathis]

@jrdh Here are some guidelines for tombstone pages: https://support.datacite.org/docs/tombstone-pages including some ideas for what metadata to display.

We also recommend updating the DOI state to Registered, more info on that here: https://support.datacite.org/docs/doi-states.