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).
I agree that reuse is bad. It also assumes that the dataset has already been used before, which we don't necessarily know!

I'm with David that *Apps* is actually the best name for this:

* It's the only name suggested that doesn't sound really weird
* It's what most sites surveyed used
* It's familiar - people are used to app stores (and many app stores also contain other content like magazines, books etc)
* Most data reuses on the sites surveyed were apps, very few other kinds of things anyway
* Technically "application" can mean "application of data" so it (sort of) works. I realise that people won't think this when they first see the word apps, but on the showcase page we can put a one-sentence description to the effect of "This is a showcase of things people have done with our datasets: apps, visualisations, articles, or any other applications of the data..."

We have a history in CKAN of naming things very poorly by attempting to cover all the edge cases. For example when you add a data file to a dataset in CKAN, you can upload a file to CKAN, link to a file hosted somewhere else, _or link to an API rather than a file_. So instead of data files we called them _Resources_, which is ridiculous. The linking to an API option isn't even shown in the web interface anymore.