Convert horde from global $conf to HordeConfig #157

This is a low hanging fruit.

Old Pattern: global

Pattern a:

global $conf;
if (empty($conf['holidays']['enable'])) { // whatever}

Pattern b:

if (empty($GLOBALS['conf'][holidays]['enable'])) { // whatever}

New Pattern: HordeConfig

At the top of file:

use Horde\Horde\HordeConfig;

At the place where you found the pattern:
$config = $injector->get('HordeConfig');
if (empty($config->get('holidays.enable'))) { // whatever}

PRs for individual or few conversions are accepted, you don't need to submit a large bulk transformation PR.