add embedded model support to loaddata / dumpdata

[timgraham]

If an app contains an subclass of EmbeddedModel, dumpdata crashes:

$ python manage.py dumpdata embed --settings=mysite.mongo --traceback
[Traceback (most recent call last):
  File "/home/tim/code/mysite/manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/home/tim/code/django/django/core/management/__init__.py", line 442, in execute_from_command_line
    utility.execute()
  File "/home/tim/code/django/django/core/management/__init__.py", line 436, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/tim/code/django/django/core/management/base.py", line 413, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/tim/code/django/django/core/management/base.py", line 459, in execute
    output = self.handle(*args, **options)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/tim/code/django/django/core/management/commands/dumpdata.py", line 269, in handle
    serializers.serialize(
  File "/home/tim/code/django/django/core/serializers/__init__.py", line 134, in serialize
    s.serialize(queryset, **options)
  File "/home/tim/code/django/django/core/serializers/base.py", line 109, in serialize
    for count, obj in enumerate(queryset, start=1):
  File "/home/tim/code/django/django/core/management/commands/dumpdata.py", line 217, in get_objects
    queryset = objects.using(using).order_by(model._meta.pk.name)
               ^^^^^^^^^^^^^
AttributeError: 'EmbeddedModelManager' object has no attribute 'using'

If ignoring embedding models, embedded models incorrectly have only their string representation dumped:

$ python manage.py dumpdata embed --settings=mysite.mongo --exclude=embed.person --exclude=embed.address
[{"model": "embed.book", "pk": "67913fe312e1777e03b35af4", "fields": {"title": "Runaway Jury", "author": "Dan"}}]