point out existing (possibly) duplicate published artifacts on import

[carboxylman]

If user inputs a URL of an existing, published artifact, user should be warned and given a link to see if their goal is already met. This can be a false negative if user is reusing said URL for an artifact of different type, but those cases should be rare.

[carboxylman]

My thinking on this is that the backend POST /artifacts that accepts an import URL (etc) will expand to include another parameter, force. If the URL is already imported and published, we will warn in this case and point to the existing artifact, and frontend can set force=1 if the user says, yes I really want to import. Straightforward, I think?