#1866: support e-shuushuu source redirection

* currently, e-shuushuu already redirects some posts to their landing pages (http://e-shuushuu.net/image/?image_id=$id), particularly those ending in 'jpg'
* for some reason, e-shuushuu doesn't natively support other formats even the uppercased 'JPG'
* the following lines aim to address this issue to redirect to http://e-shuushuu.net/image/$id
* http://e-shuushuu.net/image.php?mode=view&image_id=$id is also a valid alternative but is not used