Presentation of "dead" links in rendered HTML

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As of #51 , we will have basic rendering of a sphere as HTML in place. We don't currently have a treatment for links that go nowhere. In the Subconscious app, there is no such thing as a link that goes nowhere, so this isn't generally a problem. However, we don't have any notion of an equivalent behavior for static HTML. We could build something similar - like a dynamic 404 page of some kind - but a simpler solution might be to just cross out links to nowhere. Discuss!