[EXPERIMENTAL] Allow to render translations fields separately

[Senen]

Sometimes we want to render the translations fields separated from each other, with not translatable fields or any other HTML component in between. Unfortunately, this was not possible because the hidden fields :_destroy and :locale were rendered for each translatable_field call resulting in duplicated fields for each translation and invalid HTML. So now we ensure to render each translation hidden_fields once per form.

In addition, the form fields for the same translation had a different index when using more than one translatable_fields block. So now, the index we use for translation fields is the position the translation locale occupies in the application's array of available languages.

This is an example of what we want to do:

    <%= f.translatable_fields do |translations_form| %>
        <%= translations_form.text_field :title %>
    <% end %>

     Some HTML here

    <%= f.translatable_fields do |translations_form| %>
        <%= translations_form.text_area :description %>
    <% end %>

The output for the previous code before this patch was:

    debate[translations_attributes][1][_destroy]
    debate[translations_attributes][1][locale]
    debate[translations_attributes][1][title]

    Some HTML here

    debate[translations_attributes][33][_destroy]
    debate[translations_attributes][33][locale]
    debate[translations_attributes][33][description]

Now, translation hidden fields are not duplicated and the index is the same for the same translation fields:

    debate[translations_attributes][1][_destroy]
    debate[translations_attributes][1][locale]
    debate[translations_attributes][1][title]
    debate[translations_attributes][1][description]