XWIKI-9737: The user XWiki.Admin is still created in subwikis.

* Add xwiki-enterprise-ui-admin-user as an embedded extension of xwiki-enterprise-ui-mainwiki-all.
Merge pull request #54 from gdelhumeau/XWIKI-9737

XWIKI-9737: The user XWiki.Admin is still created in subwikis
* Add xwiki-enterprise-ui-admin-user as an embedded extension of xwiki-enterprise-ui-mainwiki-all.