Only support live preview for known subset of HTML document extensions.

Safe fix for #3403.

A better fix would be to remove the side-affecting code in ``CSSDocument`` that sets up ``CSSAgent``, but that fix would be far more invasive. This doesn't quite get us back to Sprint 22 behavior where we would infer an index.html from a CSS file in the same directory, but it at least shows a warning that a file must be a HTML file (or a server HTML file) in order for live preview to work.

The original bug #2033 to correctly determine the default file became a user story https://trello.com/c/gbBtpARq.