* Moved htmlEscape from mediawiki.util.js to mediawiki.js so that it can be used...

* Moved htmlEscape from mediawiki.util.js to mediawiki.js so that it can be used in the loader.
* Added mediaWiki.html.element(), which provides a safe HTML construction function similar to Xml::element().
* Used element() in various places in mediawiki.js. Fixes escaping issue noted on CR r75170.
* Profiled the new mediaWiki.html.escape() at 1.4 MB/s for special characters and 154 MB/s for non-special characters on my humble laptop. Hopefully that's fast enough to convince Trevor that escaping is unlikely to be a significant component of page render time.
* Profiled mediaWiki.html.element() generating style elements with Cdata at ~17us per iteration. For comparison, $('body').append('<div/>') takes 200us.