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

* I have added a MultiWikis page in the General section of the Administration
* In this page, you have a message if workspace-template is not installed.
* In this page, you can set who has the SUBWIKI_CREATION right.
* The template rightsUI.vm has been modified to handle this new page.