Allow admin users to remove translations

Pivotal story: https://www.pivotaltracker.com/story/show/44791593

This allows admin users to delete translations for:

* Editions (currently only Worldwide Priorities)
* Worldwide Organisations (which were Worldwide Offices)
* World Locations

The workflow for Worldwide Priority translations is similar to that for Supporting Pages i.e. you cannot delete a translation for a Worldwide Priority which is currently published.

The UI for deleting a Worldwide Priority translation follows the precedent set by the UI for editing them in that no intermediate Translation "show" page exists and the "Edit" and "Delete" buttons appear in the Translations tab for the Priority. I think this makes it simpler to administer the translations, but @edds might not like my changes to the markup.