Stop using private OC_Group class

[PVince81]

Once owncloud/core#27341 is merged for 10.0, some auth related apps will stop working because they are using private APIs:

grep result:

django_auth/appinfo/app.php:OC_Group::useBackend( new OC_GROUP_DJANGO() );
user_saml/lib/hooks.php:                $old_groups = OC_Group::getUserGroups($uid);
user_saml/lib/hooks.php:                                OC_Group::removeFromGroup($uid,$group);
user_saml/lib/hooks.php:                        if (!OC_Group::inGroup($uid, $group)) {
user_saml/lib/hooks.php:                                if (!OC_Group::groupExists($group)) {
user_saml/lib/hooks.php:                                        OC_Group::createGroup($group);
user_saml/lib/hooks.php:                                OC_Group::addToGroup($uid, $group);

Please adjust accordingly using \OC::$server->getGroupManager().

CC @DeepDiver1975