Allow for graceful shutdown of heroku apps

At the moment, we nuke a heroku app as soon as the branch is merged.

However, this can make going back and testing for regressions or other edge cases difficult if that app now no longer exists.

What we could do instead is spin down the app, so we won't pay for it, but it will stay in the list of apps and could be spun up again. We could then have a secondary process that otherwise cleans them up independent of a merge.