- Heroku reads the Node version from package.json. We're about to upgrade to Node 8 so this change to `engines` is only pre-emptive. It won't have any effect on the production servers.
- The production deploy scripts were missing the frontend files. This fixes that.
- This modifies the build setup to allow `BASE_URL=/`, which makes all requests relative to the page itself. That simplifies deploying the "debugging" frontend to the production servers, and makes it easy to host the frontend on Heroku.