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.