Configuring security email needs attention

[krassowski]

Currently some security group members receive emails from security@jupyter.org and some (minority?) from security@ipython.org. It appears that some mails sent to security@ipython.org do not get delivered at all, which is a dire issue.

1. Can we make sure everyone who needs access has access
2. Can we update the SECURITY.md and jupyter.org to point to security@jupyter.org if that is to be used going forward?