Add import_backdrop_data management command

This takes a raw JSON dump of the backdrop 'buckets' collection and
creates DataSet, DataGroup and DataType models as appropriate.

See: ``python manage.py import_from_backdrop --help``

Note the ``--without-backdrop`` option which prevents Stagecraft from
asking Backdrop to create the collection.

Story: https://www.pivotaltracker.com/story/show/66132542 [#66132542]
Merge pull request #53 from alphagov/import-backdrop-dump

Add import_backdrop_data management command