Repair locale files by moving non-plural-form keys out of pluralised translations

This repairs a long standing issue with the locale file structure.

Pluralised translation terms such as those under "document.type" should only ever contain pluralisation keys (e.g. `:one`, `:many`, `:other`), otherwise the [export code](https://github.com/alphagov/whitehall/blob/master/lib/whitehall/translation/exporter.rb#L72-L80) cannot recognise them as pluralised terms and therefore doesn't generate the correct plural forms in the locale files it exports. Non-pluralised keys were added to the locale files [here](https://github.com/alphagov/whitehall/commit/c936c82) and [here](https://github.com/alphagov/whitehall/commit/35877e5), which has resulted in pluralisation keys being lost and the locale files getting mangled every time new keys were added and the files were are regenerated.