CoverArt thumbnails in torrent lists, closes #32

* Added imagetools::cover_thumb to handle the html and ::process
* Added to group and non-group locations in torrents/browse.php
* and artist.php
* CSS fixes
* Larger thumbnail popup on mouseover image as well as title, full
  image onclick

static/styles/sugoi/style.css

@@ -1137,6 +1137,9 @@ ol {
 .group_info > a:last-of-type {
   text-decoration: underline;
 }
+.group_image {
+  padding-right: 1em;
+}
 
 .recent_snatches_images, .recent_uploads_images {
   position: relative;