Add ``/blog`` URL path to HTTP GET monitoring system.

[nathan-at-least]

We're adding HTTP GET monitoring/alerting in #270. That ticket is for an "MVP" version which just checks GET /. Once that is working, this ticket can be closed by augmenting that alerting system to check both / and /blog.

This ticket is a spinoff of this comment.

[exarkun]

IOW, this would be monitoring configuration for an alert that tells us if the blog goes down. A new blog thing is coming so it might not actually be /blog (or it might be), but the idea is the same.