Django-admin-sortable2

- Fix [\164](https://github.com/jrief/django-admin-sortable2/issues/164):
TypeError when `display_list` in admin contains a callable.
- Fix [\160](https://github.com/jrief/django-admin-sortable2/issues/160):
Updated ordering values not getting saved in `TabluarInlineAdmin` / `StackedInlineAdmin`.

- Fix [\162](https://github.com/jrief/django-admin-sortable2/issues/162):
In model admin, setting `actions` to `None` or `[]` breaks the sortable functionality.

- Fix [\159](https://github.com/jrief/django-admin-sortable2/issues/159):
Make stacked inline\'s header more clear that it is sortable.

- Fix [\155](https://github.com/jrief/django-admin-sortable2/issues/155):
Sortable column not the first field by default.

- Fix [\147](https://github.com/jrief/django-admin-sortable2/issues/147):
Use current admin site name instead of `admin`.
- Fix [\122](https://github.com/jrief/django-admin-sortable2/issues/122):
Columns expand continuously with each sort.

- Fix [\139](https://github.com/jrief/django-admin-sortable2/issues/139):
Better call of post_save signal.