Store each translation as a separate key in elasticsearch

Rather than storing language objects in elasticsearch (which was what I did originally in #45) we simply flatten out any language objects so they take the form `{field_name}_{language_code}`, e.g. `name_en` or `name_en-gb`. This preserves backwards compatibility, so you can still do search queries like `name:Chris` and it will correctly return records who's name (in the default language) is Chris.