Add crossfade view on diffs page

[lencioni]

Split out from #61.

We want to provide tools for people to easily interpret their diffs. We already have side-by-side, swipe, and an LCS diff view, but some diffs may be easier to interpret with a crossfade view. This would be both images positioned on top of each other, and a slider that changes the opacity. It would be useful to have a way to toggle between A and B fully opaque, to quickly flip back and forth.