iterate over members of whitelisted github orgs
iterate over members of whitelisted github orgs

The default scope for Github auth tokens does not give high enough
privileges to list the organizations of which they are a member [1],
so instead we list the members of all the whitelisted organizations,
and check whether the user is present there.

[1]: https://developer.github.com/v3/orgs/#oauth-scope-requirements

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