Pages should have the Summary Property

Summary: That is, Summary should be a standard Property on Pages.

The challenge here is that Pages are Simple Things -- and Simple Things are the root of the user Model hierarchy. So simply adding Summary to Simple Thing would add it to everything, and I don't think that's desireable.

I am beginning to regret getting rid of the "redundant" Simple Page type. We could re-create that (in which case we'd still need to support Simple Things as Pages for a long time). Or we could add Summary manually when a Simple Thing is created. Or we could make it very easy to add it to any Thing, in the Editor.