Convert UI to ReactJS

The UI should look almost identical to how it did before, but with a couple fewer bugs and a few more features.

The main new feature is that webdiff now shows the full list of files, which can be collapsed to a dropdown if desired.
It always renders a `<DiffView>`, it may or may not render the `<ImageDiffModeSelector>` depending on whether it's showing an image diff. So I think the TODO above does make sense. I'll do it tomorrow.