Create Basic Example of using auth_plug in a Phoenix App

[nelsonic]

Create a super-basic step-by-step example using the auth_plug dwyl/auth_plug#2
and a DWYL_API_KEY dwyl/auth#42 to handle all auth requests.

• /admin route should be protected by the auth_plug
• unauthenticated request should be redirected to Auth App https://github.com/dwyl/auth
• once authenticated the browser should be redirected back to the auth-plug-example.herokuapp.com > https://dashboard.heroku.com/apps/auth-plug-example/deploy/heroku-git
• auth_plug should create a Phoenix Session so the person can refresh the page, close the browser tab and return to it without loss of their session.

This issue depends on the other two issues mentioned above,
but it's it super instructive to build the Example/Demo app while you are building the modules
because it informs the interface for those modules.

[nelsonic]

I've got the auth_plug working well. ✅
Next: Extract values from conn.assigns and display them in a template.

[nelsonic]

Setup GIF:

End result:

Boom. 💥

[nelsonic]

Working on Heroku: https://auth-plug-example.herokuapp.com/admin