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.
This solves issue in two separate places:

``BaseInlineFormSet`` has ``save_as_new`` argument in constructor and initial ``FileField``s can (and should IMHO) be set when constructing individual forms.

Admin ``add_view`` when used with ``save_as`` would construct form from request ``POST``. Since, in this case, file fields would be empty we need to copy their initial values from ``original`` (or source) instance.

Does it sound logic?