@dzucconi: Admin can feature articles to artist & artwork pages

This adds an admin section with featuring. This came with some refactors including adding a similar orchestration endpoint for artists on the API in preparation for autocomplete artist search. Soon we'll have two levels of featuring so I'll add `primary_featured_artist_ids` and `primary_featured_artwork_ids` fields, but for now this gives it feature parity with what exists for posts currently.

![cap](https://cloud.githubusercontent.com/assets/555859/5017972/49a9d48a-6a80-11e4-823d-ef0d1dfda195.gif)