DataSet tests for integration with backdrop_client
Use django transactions to commit to db
  - DataSet save method now opens a transaction
  - If creation in Backdrop fails, we roll back the entire transaction
  - Thus, a 'save' encapsulates/requires a model save in Stagecraft *and* Backdrop
  - Tests updated to test transactions
  - New property 'is_capped' on DataSets
Use updated django ‘atomic’ method