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 didn't mean to be!

You know that thing where IDEs aggressively autocomplete files and symbols

![](http://f.cl.ly/items/2b1W2e1l0O012Q440x00/Screen%20Shot%202014-09-16%20at%205.34.26%20PM.png)

![](http://f.cl.ly/items/2A1S1r1O451u1I420p0N/Screen%20Shot%202014-09-16%20at%205.35.31%20PM.png)

It just kind of clutters things.