Display links to available translations of an edition

* We are displaying links to all available translations, but it is not
currently possible to create translated editions in the admin interface, so
the links will never show up.

* We are using the "raw" locale name as the link text and not the
human-friendly language name for the translation.

* We are showing links to *all* available translations including the current
translation. We plan to "disable" the link to the current translation in a
future commit.

* We've had to add an es.yml file, containing the format of the long_ordinal
date, to avoid our viewing-published-international-priorities feature from
exploding with the following error:

    translation missing: es.date.formats.long_ordinal (ActionView::Template::Error)
Display native language names for translation links

* We have added a Locale class which uses I18n to lookup the native language
name for a given locale. We decided that the language name is itself something
that can be translated into different languages and so we've given it a bona
fide key and not stored it in I18n metadata i.e. we did something slightly
different from this suggestion [1].

[1] https://gist.github.com/henrik/276191
Only link to other edition translations

We don't think it makes sense to link to the current translation of the
edition you're viewing.
Merge branch 'links-to-available-edition-translations'

We now display a set of links to available translations of any document type.
Currently, the only way to create an alternative translation is through the
console. The related admin functionality to create translations will come
later.