Fixed #14760 - Admin inlines with file/image field fails to save_as

https://github.com/django/django/pull/697 reworked for master.

Refactored BaseInlineFormSet and ModelAdmin to copy initial
values (file name) for fields of FileField or it's subclasses.

'physical' file do not get duplicated.
We could extract common functionality to django.forms.models.copy_initial_file_fields that could then been used both in ``BaseInlineFormSet`` and ``add_view``.

    def copy_initial_file_fields(form, original):
        """Copies initial file fields from original"""
        for field_name, field in form.fields.items():
            if field_name in form.initial:
                continue
            if not isinstance(field, FileField):
                continue
            form.initial[field_name] = getattr(original, field_name)