Automatically deploy website

[jhenstridge]

We currently deploy the website when the Ansible playbook is run. Ideally we'd deploy the website more often than the playbook is run though.

There is an example of building the website in the playbook. It's basically just running hugo on a git checkout, then rsyncing the results over to the web server directory.

Triggering it could be set up in one of two ways:

1. simple: run a cron job to do a git pull each night, and rebuild if the head revision changes.
2. hard: set up a web hook that Github can trigger when new revisions are pushed.