Turn off $wgHtml5 by default

Turn off $wgHtml5 by default

I discovered that the HTML 5 doctype does in fact trigger standards
mode in all major browsers . . . but the old XHTML 1.0 Transitional
doctype we were using did *not*.  It triggers "almost standards mode" in
some browsers.  The difference is causing some rendering issues that
will have to block deployment for the moment.

See r53034, r53035, r53136.