Use hashed filenames for assets

[jamesvandyne]

Carryover from #66 . We've now got webpack building 3 different files: app.js (JS for the admin), public.js ( JS for the public site), and style.css (overall styles). These assets sometimes get cached by browsers, which can cause new functionality to not work until there's a hard reload.

Output files should include their hash so we can avoid caching issues. Once using hashes, integrate django-webpack-loader so django knows where to find the built files when running collectstatic.