Fixed #23444 -- Prevented unnecessary ContentType object creation when using inlines

ContentType object is mistakenly created for `through` model of M2M relation when it is used in inline.
The message should be a bit more descriptive. Something like, "AdminHelperForm.original_content_type_id is deprecated and will be removed in Django 2.0. If you were using the attribute to construct the "view on site" URL, use the absolute_url attribute instead."