Public website: GitHub connect

[gregberge]

As a user, I can connect using a button with GitHub logo.

Motivation

We need an authentification method and we want to integrate to GitHub first.

Detail

When I am a new user, an account with my information is created and I am logged in for one month.
When I am not a new user, my account is updated and I am logged for one month.
If I am not logged, I see a login button in the header, if I am logged, I see a logout button.
When I click on the logout button, I am logged out.