XHTML files have no syntax highlighting

[peterflynn]

1. Open an .xhtml file

Result: no code coloring

I know technically XHTML is a slightly different format from HTML, but it's close enough that it feels like maybe we should support it sooner than other formats.

CodeMirror already has general XML highlighting, so we could use that -- but it wouldn't highlight embedded JS and CSS. To do it right, we'd want an "xhtmlmixed" variant of the current "htmlmixed" mode (we could just extend "htmlmixed" to take a flag for this).

[njx]

QRB marking starter bug. Let's just hook up .xhtml to use the htmlmixed mode.