Cleanup of the layout's HTML

* Remove [useless type attributes](http://www.w3.org/html/wg/drafts/html/master/scripting-1.html#script) for `script` and `link` tags
* Change charset declaration to [the HTML5 one](http://www.w3.org/International/questions/qa-html-encoding-declarations#html5charset)
* [Remove XML-form self-closing tags](http://google-styleguide.googlecode.com/svn/trunk/htmlcssguide.xml?showone=Document_Type#Document_Type)