Include `available_translations` in links

Automatically generate a links list of `available_translations`. This is based
on grouping content items using the content_id and locale of each content item.
Include `self` in available_translations, and order correctly

This is a convenience for front-ends which will use this list to generate a
language switcher menu.

We also sort the list alphabetically by locale code, so that the list is always
in a stable order.
Add end-to-end spec describing available_translations
Documentation of locales and available_translations

More explanation of how translations of content items should be
stored in the content store.

Documentation of `available_translations` links.

Documentation of `locale` field appearing in links hash.
Add `available_translations` to output_examples

Also add `locale` to all related links
Further documentation revisions
Prevent posting items with `available_translations` set

This link type is automatically generated so you should not post items
containing it.
Clarify representation of content items in different contexts
Merge pull request #82 from alphagov/include-available-translations-in-links

Include available translations in links