Change new datasets to default to "Private"

[drw]

When creating new datasets through the web interface, the "Visibility" parameter defaults to "Public" under the new DRUF (Dataset Resource Upload First) workflow. This also results in the dataset getting published after resources are added, without even letting the dataset creator edit any of the metadata.

Switching the default to "Private" would return to the previous default and provide a safeguard against publishing before the dataset is ready.