Remove or use node is_public "denormalization"

The node form for an OG-managed non-group content type provides a checkbox for each group into which the node should be placed, plus 1 checkbox that controls whether the node will appear to be public with respect to all groups. The information model implied is that 'publicness' is a property of the node, and this will affect visibility in all groups.

On the other hand, the og_ancestry table is where the is_public field is stored, so it potentially can have a different setting for each group the node is posted in. I stumbled over this bit of denormalization while writing a tiny module to make posts not affiliated with any group be private, and was hoping to use the existing OG "public" flag to do so. The difficulty this schema creates is that, for nodes not in a group, there's no public flag at all.

I think we should either fix it (i.e. remove denormalization) or feature it (i.e. let people assign publicness per group).

It seems like this may be a different perspective on #172099.