Add .fieldBox styling for django 2.1+

[leibowitz]

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The field-box class has been renamed to fieldBox in django 2.1, therefore the style doesn't work from django 2.1 onwards. This PR adds fieldset .fieldBox to the style, to bring compatibility with django 2.1+

Reference:

The admin CSS class field-box is renamed to fieldBox to prevent conflicts with the class given to model fields named “box”.

https://docs.djangoproject.com/en/3.0/releases/2.1/#miscellaneous

[pyghassen]

@leibowitz It's actually no need to merge this pull request, because that code already exists on Django https://github.com/django/django/blob/2.1/django/contrib/admin/static/admin/css/forms.css#L180, that's why actually nobody complained, unless you have that specific css class customized in your project.

[leibowitz]

might be included in Django forms, but it isn't in django-nested-inline forms https://github.com/s-block/django-nested-inline/blob/95819fc1d240225b351fb200fe60d97e37f8f2b3/nested_inline/admin.py#L50

I wouldn't make a PR for an issue I'm not having :)