add a player preference for default chat privacy

[cgolubi1]

Players should have a preference for whether chat in their new games defaults to public or private.

This should override the global default (see #3050).

It may be possible for other new-game settings to override this one - for instance, we may want to allow for public chat tournaments (e.g. story tournaments) whose preferences override the per-player preference. So what we say about this setting on the preferences page should reflect that.