By default, I should get comments on Things I created

Summary: This is a subtle tweak to the comment rules, but makes sense -- if I created this Thing, I probably want to know what people are saying about it. See, for example, this Issues Space: users who add Issues probably want to be notified of their comments.

This is finally possible, since we finally have a concept of "who created this". But it might require enhancing Thing to carry that information along, and it will need to be optional since we don't have it for older Things.