add organization whitelisting to GitHubOAuthenticator
add organization whitelisting to GitHubOAuthenticator

This is analogous to the team whitelisting already present
in BitBucketOAuthenticator

yuvipanda/mwoauthenticator,jupyter/oauthenticator,minrk/oauthenticator,NickolausDS/oauthenticator,jupyterhub/oauthenticator,enolfc/oauthenticator,maltevogl/oauthenticator,yuvipanda/mwoauthenticator,jupyter/oauthenticator