FU r101117: removed cURL thumb handler code and made thumb_handler.php a thin wrapper...

FU r101117: removed cURL thumb handler code and made thumb_handler.php a thin wrapper around thumb.php
* Moved original URL fetching code and parameter extraction code to thumb.php
* Made use of local repo URL and hash settings to avoid extra config code
* This makes it easy to add hooks for extensions/config to alter behavoir (ExtractThumbParameters hook added)
* Added FileRepo::getHashLevels()