Investigate if we can add some sort of smoke test to revert a Heroku deploy if the app is not even starting up

[anaulin]

This has happened a couple of times in the last few years, that there after a deploy the app is no longer starting up and it takes us a bit of time to notice this.

There might be a Heroku plugin or some script or tool that we can use to try to make sure that at least the app is up after a deploy (e.g. just curl app.doubleunion.org and make sure it is returning the app) and force Heroku to rollback the deploy if the app is not app.

Barring that, maybe we can use a ping service (or our own script) to alert us if the app is down.