stage CILogon cert to spawners
stage CILogon cert to spawners

- store certs in private dir (a la .ssh)
- add CILogonSpawnerMixin for staging cert to users
yuvipanda/mwoauthenticator,luisfdez/oauthenticator,santi81/oauthenticator,NickolausDS/oauthenticator,willjharmer/oauthenticator,jupyterhub/oauthenticator,maltevogl/oauthenticator,yuvipanda/mwoauthenticator,jupyter/oauthenticator,minrk/oauthenticator,jupyter/oauthenticator,willjharmer/oauthenticator,enolfc/oauthenticator,luisfdez/oauthenticator,santi81/oauthenticator