Pure CSS two column scrolling for collection building

@jcoyne - could you want to pull a copy of this branch and see if you can break it in any way (on a tablet, supper narrow viewport, really big viewport if you have an external monitor, phone, etc).  I think it addresses all the issues we've seen to-date and it should be cross-browser compatible (I have to test it on IE yet, but Safari, Firefox, and Chrome all work)  Firefox seems like it may have some difficulty figuring out the scroll target, but I think that's ok - I'm not sure we tested the javascript version on firefox either.

If you like Brian's javascript version better, we can stick with that, but it stutters really badly when scrolling for me in Safari.