Allow to register "private" data.

[Xarthisius]

Rationale

There are two different types of "private" data we have encountered:

1. Data that's yet not published (e.g. ODUM's curation workflow)
2. Published data that's available by request or via API key (e.g. some datasets in DV, Kyle B's archaeology Tale)

In 1) I'd propose to register data in a private folder that's owned by admin and read-only for user doing the registration to avoid accidentally exposing data info. In 2) data can be registered as usual as metadata (file names/sizes etc) is publicly available either way, it's only the content that's access protected.