Add unlabelled dataset in YAML

[andreassot10]

In YAML, replace dataset_unlabelled: text_data with dataset_unlabelled: <unlabelled_data> when unlabelled data become available.

See #43.

[andreassot10]

Just to add that this relates to

The-Strategy-Unit/experiencesdashboard#57

and

The-Strategy-Unit/experiencesdashboard#56

in experiencesdashboard.

@ChrisBeeley may want to do a similar hack for pxtextminingdashboard to upload unlabelled data as they come.