setup script can easily be run twice

[ndp]

It's easy to run the setup script twice, which then messes up your heroku configuration, and basically makes you start over if you don't want to tweak various settings. See https://techtonicaorg.slack.com/archives/C0259C7V9DG/p1633025490298200?thread_ts=1633021363.288300&cid=C0259C7V9DG

Idea: have a quick check at the beginning of the setup script that blows up if it has already run. One check would be to see if git remote | grep heroku returns something. I'm sure there are other ideas.