Swap out secrets.py for env vars

I've already set the relevant secrets as config vars on the app.

Like John said in the email yesterday, you can get a dump of them by typing `heroku config -s > .env` inside the project directory. Foreman is _supposed_ to add stuff in .env to the environment before running anything in the procfile.

I don't have python3 setup locally to test this commit out, give it a spin before pushing to heroku.