Reorder management command per #188

[shuckc]

The great work from #188 by @flupzor with a couple of small fixups

[shuckc]

Runing the reorder_model management command manually over all of our test models now fails only with tests.Answer since we lack support for multiple fields in order_with_respect_to:

~/django-ordered-model$ django-admin reorder_model --pythonpath=. --settings=tests.settings --skip-checks tests.Answer
Traceback (most recent call last):
  File "/home/chris/django-ordered-model/venv/bin/django-admin", line 8, in <module>
    sys.exit(execute_from_command_line())
  File "/home/chris/django-ordered-model/venv/lib/python3.8/site-packages/django/core/management/__init__.py", line 419, in execute_from_command_line
    utility.execute()
  File "/home/chris/django-ordered-model/venv/lib/python3.8/site-packages/django/core/management/__init__.py", line 413, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/chris/django-ordered-model/venv/lib/python3.8/site-packages/django/core/management/base.py", line 354, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/chris/django-ordered-model/venv/lib/python3.8/site-packages/django/core/management/base.py", line 398, in execute
    output = self.handle(*args, **options)
  File "./ordered_model/management/commands/reorder_model.py", line 54, in handle
    self.reorder(model)
  File "./ordered_model/management/commands/reorder_model.py", line 58, in reorder
    order_with_respect_to = get_order_with_respect_to(model)
  File "./ordered_model/management/commands/reorder_model.py", line 13, in get_order_with_respect_to
    assert (
AssertionError: re-ordering with more than 1 fields in order_with_respect_to is not supported

I would rather keep this as an NYI since there is (great) utility in merging this.

[imomaliev]

I would rather keep this as an NYI since there is (great) utility in merging this.

I think this is ok for now

[shuckc]

I've pushed the bulk_update patch discussed #240 (comment) to branch origin/patch_reorder_after_dj22 so if anyone needs it and is on Django > 2.2 they can apply it.