(UPDATE) prevent of using i18n field as form field

[dfirst]

Why i did it.

1. We have this line https://github.com/zostera/django-modeltrans/blob/master/modeltrans/translator.py#L138, currently flag (editable=False) did nothing because it isnt set to TranslatedVirtualField.
2. I think in case if we need active language model form we need to force user to use solution like ActiveLanguageMixin.
3. Also i18n virtual form field can be unpredictable in case of (required=False) because its always (blank | null = True).

Alternative of this solution can be using some kind of lazy form field.

What do you think?

[coveralls]

Coverage remained the same at ?% when pulling 8a629ab on dfirst:UPDATE_prevent_using_i18n_virt_field_as_form_field into 862d4f5 on zostera:master.