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.
does this live in `util`? I am getting an error running `python webdiff/app.py ...` because it seems like this should be `util.annotate_pairs`. Am I missing something?