Django-admin-sort

- Fix CSS classes change introduced in Django-2.1.
- Prepared to run on Django-4.0.
- Ditch Travis-CI in favor of GitHub Actions.

- a major version is created when the software reached a milestone and is
feature complete

- Fix [\207](https://github.com/jrief/django-admin-sortable2/issues/207):
Last item not displayed in stacked- and tabular inline admins, if model doesn\'t have add permission.

- Add support for Django-3.1.

- Fix [\241](https://github.com/jrief/django-admin-sortable2/issues/241):
Move items when the order column is not first.
- Fix [\242](https://github.com/jrief/django-admin-sortable2/issues/242):
Bulk move when sorting order is descending.
- Fix [\243](https://github.com/jrief/django-admin-sortable2/issues/243):
Bulk move to last page when it is too small.
- Refactor aggregates to use Coalasce for empty querysets.

- Add support for Django-3.0.