Persist and load history using a YAML file, so that server restarts don't lose pushed data

This change adds an `at_exit` hook that saves all the event history in a YAML file. When dashing starts, it loads the events from this file if it exists.

We push data to dashing every hour, and don't want to lose this pushed data whenever dashing is deployed. (We host it on our own server, not Heroku.)