Cleanup on aisle two in Site Updates

Just doing a few style fixes. The one you'll probably notice is that I removed the part where images and videos can expand beyond the normal margin on large displays. It was an interesting experiment, but it doesn't take into account the millions of different ways users like to inject their own HTML, and most of the time it just comes across looking like something on the site is broken.

So that is gone, and the margins should be consistent now.