PR: Remember private/public authentication

This PR takes care of saving the private/public authentication properly.
The main change is, that the scopes-parameter will __not__ be provided when authenticating the user with github's oauth api.

From Github's documentation:
> If not provided, scope defaults to an empty list of scopes for users that don’t have a valid token for the app. For users who do already have a valid token for the app, the user won’t be shown the OAuth authorization page with the list of scopes. Instead, this step of the flow will automatically complete with the same scopes that were used last time the user completed the flow.

After authenticating, we check whether the granted scopes are empty. If so, the user registered for the very first time and he will be redirected to the page where he can decide which rights he want to grant.

For this, a little change in ```com.scalableminds.util.auth.GithubOauth``` is necessary. I will update it, if you agree on this PR.