[wip] Use Ansible instead of a shell script to deploy the site from Jenkins.

Use it to manage the deployment and configuration of the node.js backend (#375) as well.
This is *supposed* to not add the host key if it's already in the `known_hosts` file, but from what I've seen it re-adds the key every time. Not sure what's going on there. I don't think it's a big deal because re-running the playbook is broken for other reasons (#368).