Use thumb_handler.php to handle thumbnails.

The prior implementation used a deprecated method of capturing thumbnail
configuration data manually to pass to thumb.php. That method does not support
PDF page thumbs and other possible future changes. Thumb_handler.php is the
proper method for handling such issues on MediaWiki >= 1.20 deploys.

Change-Id: Ie2d282c2607d20bfa1a675e28a512c9ca1b2d9d4