Bugfix for issue #9583

lib/private/group/metadata.php: For subadmins also return an array of groups, indexed by their GIDs.
settings/users.php: Convert array of arrays to array of GIDs before calling into OC_Group::displayNamesInGroups.

Signed-off-by: Stephan Peijnik <speijnik@anexia-it.com>
isn't `$group` an instance of `\OC\Group\Group` here?