Review how auth application works

To be able to review #231 and to make sure I understand how the application works, I'm currently reading the code to improve my understanding of it.

I'll list any questions, changes to this issue:

- [ ] Remove Heroku related code? I don't think we're going to use the auth app deploy on Heroku, so I think it's safe to clean the repository from some of the code specific to Heroku.
- [ ] Once https://github.com/dwyl/email/issues/61 is done we'll be able to remove:
https://github.com/dwyl/auth/blob/f6a72102fd72f99ba8c42d00083e22c2fdec917a/lib/auth_web/templates/auth/index.html.eex#L66-L67