hhmi: stop allowing all users, allow only existing users

config/clusters/hhmi/common.values.yaml

@@ -50,7 +50,34 @@ basehub:
             http://github.com/login/oauth/authorize:
               username_derivation:
                 username_claim: "preferred_username"
-              # FIXME: allow_all=true is the current behavior, but its not
-              #        intentional based on discussion about how the hub was
-              #        setup.
-              allow_all: true
+        OAuthenticator:
+          # WARNING: Don't use allow_existing_users with config to allow an
+          #          externally managed group of users, such as
+          #          GitHubOAuthenticator.allowed_organizations, as it breaks a
+          #          common expectations for an admin user.
+          #
+          #          The broken expectation is that removing a user from the
+          #          externally managed group implies that the user won't have
+          #          access any more. In practice the user will still have
+          #          access if it had logged in once before, as it then exists
+          #          in JupyterHub's database of users.
+          #
+          allow_existing_users: True
+        Authenticator:
+          # WARNING: Removing a user from admin_users or allowed_users doesn't
+          #          revoke admin status or access.
+          #
+          #          OAuthenticator.allow_existing_users allows any user in the
+          #          JupyterHub database of users able to login. This includes
+          #          any previously logged in user or user previously listed in
+          #          allowed_users or admin_users, as such users are added to
+          #          JupyterHub's database on startup.
+          #
+          #          To revoke admin status or access for a user when
+          #          allow_existing_users is enabled, first remove the user from
+          #          admin_users or allowed_users, then deploy the change, and
+          #          finally revoke the admin status or delete the user via the
+          #          /hub/admin panel.
+          #
+          admin_users:
+            - colliand