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 for this one users are given a nice "submit your showcase item" form to fill in, with image uploading features and everything. Actually, even for stage 1 of the implementation we will already have created this form, for site maintainers to use. So we can reuse it here.

So for the submission methods, I'm thinking that 1. is good because it's so trivial to implement, and 3 is good, but 2 seems like kind of a bad option so we may want to skip it.