Move to .xhtml files instead of .shtml. They still use SSI.

We're now validating as XHTML 1.1 (or, at least we will be when I've tested
every page of the site to make sure it validates). I've put the necessary
frigs in the webserver so it returns the content-type as text/html if the
web browser doesn't make any claims to know about application/xhtml+xml
(which is the content type defined by the XHTML standards).