For HTML 5, drop type="" attributes for CSS/JS

For HTML 5, drop type="" attributes for CSS/JS

This time done in a nice, centralized fashion, reducing LOC for callers
even if HTML 5 is disabled.  The implementation is a new Html class,
similar to Xml but intended to be HTML-specific from the beginning
instead of half-heartedly attempting to provide generic XML services but
actually with lots of HTML-specific stuff tacked on.

As part of the new Html class, a global config option $wgWellFormedXml
is added.  It's set to true by default, but if set to false, the Html
class will drop some things that HTML 5 doesn't require, like
self-closing " />" syntax and attribute quotation marks (sometimes).