Add the ability for writers to translate editions

We've added the basic functionality to allow writers and editors to create
translations of non-published documents. The translation is limited to the
title, summary and body at present.

We've taken a lot of inspiration from @tomafro's work on adding translations
to world locations.

We've followed the pattern of allowing document types to specify which
behaviour they have and introduced the `Edition.translatable?` method. We're
currently only allowing International Priorities to be translated.

We've chosen to show the available translations in a similar fashion to
supporting pages, i.e. in their own tab and presented as a 'page' per
translation. *NOTE* The ability to edit translations will come later.
Mimic world location translation flow for editions

This is essentially a copy of the work @tomafro committed in
3e1d247261ac70d0cd2d469ffdc67b9409b7db2f.

We now have the ability to edit existing translations and add new
translations for those languages that haven't yet been translated.
Display the English text when translating an edition

This follows the pattern established by @tomafro in his work to enable
translations of world locations.
Hide the link to add/edit translations on published editions

*NOTE* We're not currently doing anything to actually prevent you adding a
translation to a published edition. This mirrors the functionality around
supporting pages, where we hide the links to create/edit them, but don't
actually prevent it at the model level.
Show the number of translations in a badge

This follows the pattern we've established for showing the number of
supporting pages and the number of attachments.
Control access to translations of editions with limited access

This follows the pattern of limiting access in the editions, edition workflow,
editorial remarks and supporting pages controllers.

Prior to this commit, *all* users would be able to create/edit translations of
an edition that had the `access_limited?` flag set.
Add govspeak markdown help to edition translation form

I think it probably makes sense to include the markdown help here, but we can
easily remove it if it's found to be a problem, particularly when it comes to
right to left languages.
Merge pull request #234 from alphagov/admin-functionality-for-translating-editions

Admin functionality for translating editions

https://www.pivotaltracker.com/projects/367813#!/stories/43569731