Security fix: fix gitlab group whitelist check
Security fix: fix gitlab group whitelist check

check group membership of users

previously, group_whitelist allowed any user who could access the groups in the whitelist,
rather than checking their membership of the groups.

Admin users received the correct behavior already

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