[wip] Set up monitoring alerts on the staging server.

This will fix #720 and #518.

- [ ] Change the notification plan at deployment-time to send email to a configurable address, instead.
- [ ] Allow `monitoring.yml` to be executed independently of `site.yml` by collecting its own facts about the relevant load balancer.
- [ ] Prompt the user for monitoring setup details in `script/deploy`. Allow an override by environment variable for those of us who run it over and over.
- [ ] Install the same monitoring setup on the staging load balancer, too.
- [ ] Figure out how to add the monitoring endpoints to the IP access list so the checks can actually work.