Add support for Nested Spaces in Group Management inside Administration

With the addition of Nested Spaces, we can now create Groups inside a nested page.

The problem is that the Groups livetable in Administration only displays page names, so this creates 2 issues:

1. Groups created in different locations/levels will be displayed on the same level
2. Groups in different locations but with the same page name will appear as duplicated in the livetable. This also includes groups created in NestedPages which will just show up as WebHome.

This is not purely a NS issue, since we would have had the same name collision if we were creating groups in different spaces. The UI wrongly assumes that all the groups are inside the XWiki space which was not true in the past an is not true now either. The UI to create groups does create groups inside the XWiki space by default, but also lists all the groups in the wiki, not only those in the XWiki space. NS simply increases the likelihood that this will become an issue.