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.
And also there is still an option to create a separate `InlineModelForm` method decorated with `cached_property`  that will do it the same things as now and raise appropriate exception (to pretened that it doesn't exist) when used without `original` for consistency.