Enabled changing the sort order manually for stacked and tabular inlines on sort field change.

Enables changing the sort order manually on sort field change
        - Top entry is 0
        - Last entry is n-1. Or any number that equals or is greater than n.
        - Sortables without a value (extra forms) gets respected. If they are at the end, they stay there.
        - Non-Number entries are being ignored
        - It does not cleans up the order field values on change (just like dragging does not do that).

See https://github.com/sehmaschine/django-grappelli/pull/423