Webpackify

Combine the several dependencies (MIDI, Base64, Base64Binary) of Song of GitHub into a single bundle using Webpack and add a few scripts to ease development.

Doing this surfaced a bug that I suspect was the origin of the comment `// weird thing with MIDI lib. vOv`. MIDI.js uses an XMLHttpRequest polyfill that expects to run in the global context. I opened a bug for the issue but used the most open of shims in Webpack to make it work for the time being: https://github.com/mudcube/MIDI.js/issues/110

Also:

* Enable gzip compression for Express, which cuts JS assets to 30% of their size. The HTML was already tiny, but it shaves a bit off there as well.
* Remove duplicate and unused soundfont directory.