Use auth service UI for authentication

[SimonLab]

Currently the Github and Google login buttons' urls are retrieved from the API and Elm is responsible for displaying them on the page. Instead we can delegate the authentication responsibility to the authentication service directly. Once loggedin the auth service should be able to redirect the user to the Elm application.

• [x] Remove the Auth page: https://github.com/dwyl/app-mvp-elm/blob/master/src/Pages/Auth.elm
• [x] Redirect to the authentication page to the auth service

[nelsonic]

@SimonLab thanks for opening this issue to refactor Auth. 👍 Please ensure there is a diagram explaining the flow (if not, please create a new slide in the Google doc): https://docs.google.com/presentation/d/1PUKzbRQOEgHaOmaEheU7T3AHQhRT8mhGuqVKotEJkM0

Auth is the one thing we have to document like our lives depend on it. 😉

[SimonLab]

To be able to test easily the changes without breaking the current authentication application, I've created a new Google OAuth application and deploy a copy of github.com/dwyl/auth-mvp/ to Heroku: https://auth-mvp-test.herokuapp.com/auth/urls

From there once the new deployed auth service is ready I can test it on localhost with the Elm application.