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).
So, all moderation is left to site maintainers? If a site maintainer has deemed a reuse submission as okay, it will appear with all datasets it is associated with. I'm okay with that as it's less work for data publishers, but I can imagine a scenario where an app is developed that uses data in an unflattering way for the data publisher (late trains) and they may not want it directly associated with their data from the dataset itself. But as @wardi suggests, perhaps that can be left as an exercise for individual sites.