Add integration for creating groups in SilverStripe to sync to LDAP

[robbieaverill]

The LDAPService has a createLDAPGroup(Group $group) method, however it's not connected via the LDAPGroupExtension at present.

It would be nice if this were possible, via configuration or default, in a similar way as LDAPMemberExtension uses LDAPService::createLDAPUser to sync members to LDAP when created in SilverStripe.

In a quick discussion with @mateusz he highlighted that it's potentially dangerous and risks an infinite loop between writing SS -> LDAP and syncing back again, but has provided some user code that can be used to do this:

$dn = sprintf('CN=%s,%s', $groupCode, \Config::inst()->get('LDAPService', 'new_groups_dn'));
$data = $this->ldapService->getGroupByDN($dn);
if (!empty($data['objectguid'])) {
    if (is_callable($logFn)) {
        $logFn(sprintf('* Using existing LDAP group "%s"', $groupCode));
    }
    $group->GUID = $data['objectguid'];
} else {
    if (is_callable($logFn)) {
        $logFn(sprintf('* Creating new LDAP group "%s"', $groupCode));
    }
    $group->Title = $groupCode;
    $this->ldapService->createLDAPGroup($group);
}

Will need further thought and discussion before implementation.