add /storage/thumb/media/:slug defroute (re-added).

I think I had a version of this route but I deleted it -- can't
remember if it wasn't needed or didn't work as intended at the
time. Anyway, this commit makes it part of the code-base (again).

The function provides the website to use the automatically generated
thumbnails -- when a user uploads the an image to /storage/media --
instead of the full-sized image. This should help reduce download
times if a list of uploaded image are several Mega Bytes  and being
viewed at once -- like an index page for example.

src/web.lisp

@@ -759,6 +759,15 @@
 			  (nera:get-storage-file :slug slug))))
       (on-exception *web* 404)))
 
+(defroute ("/storage/thumb/media/:slug" :method :GET) (&key slug)
+   (let ((storage-file (nera:get-storage-file :slug slug)))
+   (if storage-file
+       `(200 (:content-type ,(files::file-type-of storage-file))
+	      ,(storage:open-binary-file
+		"" "media"
+		(format nil "thumbnail-~a" (files::name-of storage-file))))
+       (on-exception *web* 404))))
+
 (defroute ("/storage/thumb/archive/:slug" :method :GET) (&key slug)
   (if (nera:get-archive-entry :slug slug)
       (let ((archive-entry (nera:get-archive-entry :slug slug)))