add higher res favicons for onebox

app/assets/stylesheets/common/base/onebox.scss

@@ -83,6 +83,7 @@ a.loading-onebox {
 @mixin onebox-favicon($class, $image) {
   &.#{$class} .source {
     background: image-url("favicons/#{$image}.png") no-repeat;
+    background-size: 16px 16px;
     padding-left: 20px;
   }
 }