Handle non-ASCII paths.

Rails automatically decodes percent-escaped parameters, but these are no
longer valid paths and fail both our absolute_path validation and the
parsing involved in calling out to other HTTP services. We now
explicitly re-encode the base_path and use that encoded version in the
database and when calling URL arbiter etc.
Merge pull request #83 from alphagov/encoded-urls

Handle non-ASCII paths.
Merge pull request #81 from alphagov/use-appropriate-locale-for-links

Use link in the appropriate locale
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.