XWIKI-9405: Add a "SubWiki Creation" right.

* I have added the SUBWIKI_CREATION right to the default rights set.
* I have added a check in menuview.vm to not show "add workspace" if the current user does not have this new right.
* I have given this new right to the XWikiAdminGroup by default.
* I have changed DefaultWorkspaceManager#canCreateWorkspace() to take into account the new SUBWIKI_CREATION right.