Django-admin-sort

- 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.

- Fix [\135](https://github.com/jrief/django-admin-sortable2/issues/135):
Better call of pre_save signal.
- On `./manage.py reorder ...`, name the model using `app_label.model_name` rather than
requiring the fully qualified path.
- In `adminsortable2.admin.SortableAdminMixin` renamed method `update` to `update_order`,
to prevent potential naming conflicts.