Independent featuring by language on world locations

This adds the ability to have different featured documents on the different locale versions of world location pages.

In doing this I have created a general purpose new model and user interface for administering featured items.

A `FeatureList` is a collection of `Feature`s. A `Feature` has an document, image and alt text. Note in particular that Feature references document rather than edition, so we no longer have possibility that an edition would become 'unfeatured' when a new draft is published. The Feature always shows the latest published edition of the featured document.

`FeatureList` has a polymorphic association to a featurable, so this can be extended to other types in time.

I have only applied this featuring mechanism to `WorldLocation`s at present, we can hopefully refactor other featuring code to use this in time.

In addition to these changes I also took the chance to improve the admin UI of worldwide organisations. I put the translations as a tab, rather than a separate special page.

https://www.pivotaltracker.com/story/show/45319901