[WebUI] Fix tracker icon to fit within tracker column rows

For me at least, Safari on Mac OS X, the tracker icon significantly
overflows in the Tracker column of the torrent list. (It does show the
correct size in the Trackers filter, though. Different CSS.)

This change causes it to constrain down to the height of the column and
display correctly.

Closes: #440

deluge/ui/web/js/deluge-all/TorrentGrid.js

@@ -61,7 +61,7 @@
         return String.format(
             '<div style="background: url(' +
                 deluge.config.base +
-                'tracker/{0}) no-repeat; padding-left: 20px;">{0}</div>',
+                'tracker/{0}) no-repeat; background-size: contain; padding-left: 20px;">{0}</div>',
             Ext.util.Format.htmlEncode(value)
         );
     }