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.
I have. I don't see a lot of benefit for webdiff, though -- it's definitely not a bottleneck.

I did look around for a simple Flask/JSX plugin, but (surprisingly) I didn't find anything.