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.
any thoughts on `imageDiffMode` and `changeImageDiffModeHandler` being better grouped and plumbed down inside some `imageDiffProps` object, per my rambly ruminations about ideal patterns around this from last night?