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
 * This new one add some translation keys to explain what a descriptor is (and what an alias is)
 * It also add a link to the wiki administration page
 * The code avoid HTML except when I was not able to do without it and I believe it is more clear