Githubprovider should use user/login, not user/id, for access control to be possible

I can't very well create a good access control with the current implementation since with github the user/id is a meaningless integer that nobody knows about their github account without calling github json apis. I can't ask a user to provide that integer because it isn't shown anywhere in the github UI.

In the github API the user/login field contains the github userid. Which is perfect for accesscontrol.

Find attached the proposed change.