[SERVER] Serve static assets from S3 and CloudFront

[willroberts]

When loading the browser client in the development environment, all assets are served over Express in the API process.

This results in 75MB of additional data transfer to users: 47MB for duelyst.js, vendor.js, and duelyst.css, and 28MB for additional resources (music, images, and more).

We already have code for serving these from S3 for the production environment, so we should make sure this works and make use of it.

This will reduce both cloud provider costs and CPU utilization on the API process.