Add auto-update on GitHub hook

**This pull request requires manual intervention to merge**

This sets it up so that you don't have to do all the annoying deploys yourself - instead, click "merge" on a pull request and the site will do the rest. I don't know the exact details of your deployment procedure, but you should be able to modify my guesswork to suit your needs.

1. Modify to suit your actual deployment procedure
2. Add a GitHub web hook that points to `/api/hook`

When you push to master, it'll do a git pull and a npm install, then exit. You should set up a watchdog that restarts the site when that happens, or change all that as appropriate to suit your actual needs.