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.
The better way to deal with this is to make ImageDiffModeSelector always render a selector and move the logic of whether to show it into a parent component. I think having components which may or may not render themselves is an anti-pattern.