[wip] Monitoring

I'm adding Cloud Monitoring checks to the site as per #141, but first I'll need to write the Ansible modules to create entities, checks, alerts and so on.

## Remaining Work

- [ ] Use PagerDuty for the notification plans instead of email.
 - [ ] Set up `ansible-vault` somewhere nice to store whatever secrets we need to configure PagerDuty.
- [ ] Test the notifications against a full deployment of the site, to include:
  - [ ] Changing the markup to trigger each `remote.http` alert.
  - [ ] Taking down the webservers or the load balancer entirely to trigger the `remote.ping` check.
- [ ] Hide the `ansible-playbook` invocation behind a `script/deploy` entry point, since the command-line args are getting crazier now.

/cc @rgbkrk