Create and register a main wiki owner in the DW

This will fix various right issues and workaround when using author from the XAR and it's also much safer to let the user choose the login/password of the first admin user instead of ending up with tons of wikis out there having the standard Admin/admin user which have the right to do anything.

It also mean generating a XWikiAllGroup to store the user and an XWikiAdminGroup and give this group admin right, two more thing that could be removed from XE.