XWIKI-8876: Promote the wiki-manager's XWiki.XWikiServerClass to "mandatory class" status for the main wiki
- Moved and renamed the sheet's content to be converted into a real XWiki document
XWIKI-8876: Promote the wiki-manager's XWiki.XWikiServerClass to "mandatory class" status for the main wiki
- Extracted the code that initializes the class into a MandatoryDocumentInitializer, leaving the wiki-manager XClassManager only with initializing the template which it needs for wiki creation on the current logic
- The class sheet is now a relgular xwiki document and part of the wiki-manager-ui module