db_schema: system_info.{dataset stuff} -> {dataset stuff}

[lyuyangh]

blocked by #437
part of #433

This one is a little more involved than the previous ones because dataset is an optional field. Instead of dataset_metadata_id + dataset_split, the following is stored in the database after the change

dataset: {
  dataset_id: string,
  split: string,
  dataset_name: string,
  sub_dataset: string
}

As a result, system_from_dict doesn't need to implement special logic to convert dataset_metadat_id to dataset anymore. Instead, this is handled in create_system(). The POST /systems interface isn't changed so this shouldn't break the client.

[neubig]

LGTM, thanks!