XWIKI-9412: Add a UI to handle the new "SubWiki Creation" right in the Administration.

* Move XWiki.AdminSubWikisSheet from xwiki-platform-administration-ui to xwiki-platform-workspace-ui
* Add a ConfigurableClass to it instead of hacking XWiki.AdminSheet
** Because of this, the Subwiki section will be at the bottom of the "Application" section instead of being in the first position of "General".
** It will be fixed when we will use UI Extension Point instead of ConfigurableClass in the Administration.