Showcase specification

This Pull Request refers to issue #64.

Please use this Pull Request for discussion related to the new CKAN 'Showcase' feature (aka, Related Items). **DO NOT MERGE** until the spec has settled (likely when the feature is released).
It sounds like a CKAN page needs some text to the effect of: "Have you made something with this data? [Login to share what you've done]". (This is coming out awkward because I don't have a word to use for the "thing" without calling it an app.)