Get build out of source control

[maxwellskala]

I think Heroku has hooks for running post-deploy scripts, let's have it run the Webpack build command.

[maxwellskala]

This isn't gonna be a thing until we eject, which I haven't even decided if we're going to do for the MPV. Need react-scripts to build and I suspect if we install it at the top level package.json it won't work anyways since it'll be a dir too high. Shelve for now, perhaps circle back after an actual use of this repo and a successful post-deploy build situation.