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).
Pocket is a website that lets you save articles, images or videos. In their UI they seem to just call them "items", which actually might work quite well. So we'd have:

* "Showcases" like the site-wide showcase page, and showcase pages for organizations and datasets

* "Showcase items" as the names for the individual items in a showcase, but we hopefully wouldn't need to use this very often (I'm thinking just in tooltips and on buttons like "Add Item", "Delete this Showcase Item", etc. But for the most part the words "showcase item" wouldn't need to appear very prominently anywhere.

* I would still have a short sentence somewhere on the showcase page that points out that showcase items can be different types of things like apps, articles, etc.

* If the page includes a sidebar or something for filtering the articles by type or tag, then if those tags to filter by are displayed quite prominently on the page and the tags for the main "types" of thing are promoted to the top of the list, that should also help to make it clear.

* Also when displaying collections of showcase items themselves, if each item has a few tags on it like "App", "Paper" that makes is quite clear to anyone looking at the page.

* AS @Aaron-M points out we need some clear call to action encouraging people to add their items, at that time we can make it as clear as possible: "Have you made something with data from this site? [Login to add your item to the showcase]."