CH-921: Make the list of campaigns scrollable when the orientation is horizontal

Maybe it would be less error-prone down the way if there was a common variable here - something like:
var $holder = $scrollable.length > 0 ? $scrollable : $menu;

And then on 265 - you don't need the wrapping if and it can just be $parent.prepend(element) -- and hopefully we're less likely to screw up that logic in the future.

(Not a necessary change - just a thought).