[#64] Showcase specification

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).
How would anyone navigate to a "My Show & Tell" page? People are expected to delve through hundreds of users to hit on one with a My Show & Tell page? (That also assumes that a User browse/search page is public - I don't think big sites are comfortable with that). I think basically if you are going to take content from the public you need moderation - there is no getting around that. So rather than framing it as creating 'My Show & Tell' which actually mean nothing until an admin puts them in a 'Showcase', I think you ask people to 'Add Apps', and moderation policy may show them straight away or not, with the mechanisms of a 'report abuse button' and/or moderation queue and/or spam filter.