Moved file_chmod_directory and file_chmod_file into settings system

Moved the chmod variables to the new Settings API (i.e. settings.php). The main benefit is that config files can now be made writable by both webserver and the cli user and thus all sorts of drush commands are able to work without disrupting web admin users (config commands, pm-enable, ...).

Drupal 7:

$dir_mode = variable_get('file_chmod_directory', 0775);
$file_mode = variable_get('file_chmod_file', 0664);

Drupal 8:

use Drupal\Core\Site\Settings;
...
$dir_mode = Settings::get('file_chmod_directory', FILE_CHMOD_DIRECTORY);
$file_mode = Settings::get('file_chmod_file', FILE_CHMOD_FILE);