[WebUI] Constrain large icons to fit sidebar properly

Add CSS background position and size for sidebar icons.

Without this change tracker icons that are too big, render as too big.
This restrains them to fit into the sidebar list item.

deluge/ui/web/css/deluge.css

@@ -320,6 +320,8 @@ dl.singleline dd {
 
 #sidebar .x-deluge-filter {
     background-repeat: no-repeat;
+    background-size: contain;
+    background-position: left;
     padding-left: 20px;
     line-height: 16px;
 }