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).
What is this 'curation' thing? There is no opinion required here. A 'reuse' is either ok/true/correct or it has no place anywhere on the site and should be deleted. Are you saying that there is a need for data publishers to NOT show some apps that are related to a dataset? We've not found that.