Clean up scripts

I removed a bunch of cruft from the webpack config and moved the `build` and `test` into the command instead of their own configs. Fewer places to configure things that way. Also an exercise for me to figure out how webpack works.

`process.env.NODE_ENV` is now visible to webpack, so React minifies properly.

I've also moved off BrowserSync and replaced it with the old simple static server. I never used BrowserSync's features, and it was adding to load time, and logging all kinds of nonsense when I stopped the server. If we want to use it in the future, it's got a nice proxy feature that'll work.

Running `test-mac` now explicitly calls Google Chrome in an incognito window, which means no preexisting cookies to worry about.
Super cool