Review how auth application works

[SimonLab]

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 Chore: Migrate Email Dashboard from Heroku to Fly.io email#61 is done we'll be able to remove:
  

  auth/lib/auth_web/templates/auth/index.html.eex

  Lines 66 to 67 in f6a7210

  	<!-- Wake the Email Heroku App github.com/dwyl/auth/issues/50 -->
  	<img src="https://dwylmail.herokuapp.com/pixel" width="1" />