Bug fix: OAuth2 registration

The Oauth registration was buggy for several reasons. The links used in the templates were the wrong one inhibiting the correct behaviour. The site ID configuration was wrong. This part is fixed by updating the migration script. The current system needs at least to have the ALLGO_SITE_ID set up corresponding to the actual URL of the website (http://localhost: in development https://allgo.inria.fr in production. The ACCOUNT_USERNAME_VALIDATORS was raising an error on the Oauth registration and has been removed. No known bug were introduced by doing that.

The login and signup templates have been rewritten in order to have a smoother and nicer integration in the current website. Moreover, the focus has been made on the OAuth registration (that's what users see first).

The user admin part has been modified in order to display the possibility to change the password in case the user has set up a local account on the service or the management of its Oauth connections (redirecting to the connextions.html template provided by the django Oauth plugin and integrated in the website.