Fixed the way that the CustomInlineFormset class gets used.

Rather than executing `inlineformset_factory()` in the constructor,
`SortableInlineAdminMixin` now specifies the `formset` class attribute just
like `admin.InlineModelAdmin` does it. Since users already have to put the
`SortableInlineAdminMixin` before `admin.TabularInline` in the class definition,
this means that `admin.InlineModelAdmin.formset` gets overridden, letting
`admin.InlineModelAdmin.get_formset()` do it's job as usual.

This fix is needed because the way it used to work would cause a bunch of 
important settings in the user's `XYZInline` class, like `fk_name`, to be ignored.