Add import_backdrop_data management command

MERGE https://github.com/alphagov/stagecraft/pull/52 FIRST (as this PR is rebased off it)

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]