Startup: Warn about users without site

We currently add an admin alert message when there are users without a Cooperator.

This needs to be updated to

1. Check only user accounts that are active and
2. Add a new check that all active users have a cooperator with site.isNotNull()

There is a quirk in the CT: it requires that users are associated with a Site. When there is one with site == null all hell breaks loose.