XWIKI-9687: Better sheet for XWikiServerClass.

 * The last one had problems such as:
 **  XWIKI-9881: Error when editing section on wiki descriptor page
 **  XWIKI-9828: $objTemplate.get("iswikitemplate") displayed in wiki descriptor
 * Now uses the forms standards (http://platform.xwiki.org/xwiki/bin/view/DevGuide/VerticalForms).
 * Fix xHTML validation.