rails/propshaft: Deliver assets for Rails

Propshaft is an asset pipeline library for Rails. It's built for era where bundling assets to save on HTTP connections is no longer urgent, where JavaScript and CSS is either compiled by dedicated Node.js bundlers or served directly to the browsers, and where increases in bandwidth has made the need for minification less pressing. These factors allow for a dramatically simpler and faster asset pipeline compared to previous options, like Sprockets.