Allow the GitHub and GitHub Enterprise remotes to restrict who can login
based on a user's organisation membership. This can be used as a safe
addition to open registration and also ensures that access is revoked when a
user is subsequently removed from the org. The default is not to restrict at
all.
To test that it correctly authorises a valid user, prior to me extending it.
This only tests the happy path because simulating oAuth failures is going
to be quite hard at this stage.