Abstract out the listing-detail-contentitem pattern to ICEkit

[cogat]

The pattern where there is a (publishable) django model and a fluent page that lists them, is quite common. We should provide an abstract page type to do the listing, given a model name.

(Maybe move this ticket to the ICEkit project)

[cogat]

Further, there's a pattern where we want to show a selection of a collection as a content item. Might be worth also abstracting this out at the same time, but this may not be so easily generalised.