After adding a new translation with invalid data and sending the form,
we were disabling the new translation when displaying the form again to
the user, which was confusing.
Creating a new form builder might be too much. My idea was so the view
uses more or less the same syntax it would use with Rails' default
builder, and so we can use `text_field` instead of
`translatable_text_field`.
This change forces us to use nested attributes for translations, instead
of using the more convenient `:"title_#{locale}"` methods.
On the other hand, we can use Rails' native `_destroy` attribute to
remove existing translations, so we don't have to use our custom
`delete_translations`, which was a bit buggy since it didn't consider
failed updates.
It turns out it is not necessary to downcase and underscore
locale names to use the globalize-accessor gem. The gem
will automatically underscore the locale name when defining and
calling the accessor methods.
