WL-1026 : A msg is public if it is public or its site is

Announcements made on a .anon site should be public by default.

The code uses reverse logic : a msg is not public iff it is not in a public site and it is not public itself.

This commit is twinned with that in the WL-1026 branch in wl-announcements.
I think you can just use the reference for the channel rather than explicitly depending on the Site API.