First iteration of network based i18n translations

A i18n backend which can be plugged into Rails. This will let us load
translations for strings which are in components dynamically the same
way we are loading templates. This means we can then reliably update
translations for components across the site.

As this uses the same caching engine as the component loader the
translation calls and the translations should always remain in sync.

This will need some changes to Static to serve JSON translation files.
The files which will need to be served are:

    /templates/govuk_component/translations

This should be a JSON array of different locales which are available.
For each locale that is available there should be a JSON representation
of the YAML file you would normally find in ./config/locales/:locale.yml:

    /templates/govuk_component/translations/:locale

Where `:locale` is a two letter language code.

The I18n backend can be added to the default Simple backend in rails by
calling:

```ruby
I18n.backend = I18n::Backend::Chain.new(I18n.backend, Slimmer::ComponentI18nBackend.new)
```

This will cause Rails to first try and use the strings from the
application and then fallback onto this backend. This will also enable
applications to override strings if they need to be different for some
reason.

In a component you could then call something like:

```erb
<dt><%= t("govuk_component.metadata.from", default: "From") %>:</dt>
```
Override #lookup in ComponentI18nBackup

The `#lookup` method is for loading the translation keys. By overriding
this method instead of `#translate` we retain the string interpolation
behaviour from the base i18n module.

Also refactored a few bits of code based on the observation that the
cache keys do not necessarily need to match the files or url paths
loaded from static, as long as they are consistent.

(paired with @edds on this)
Make i18n backend non-component specific
Merge pull request #101 from alphagov/translations-backend

First iteration of network based i18n translations