Add third "statistics" class of field (in addition to boolean "private" flag)

[benzea]

For GUADEC we have some statistics fields. For the workshops I need to be able to view the private workshop registration. This means that I will also see the statistics fields with private data that nobody (except the person themselves) should ever associate to the name.

My suggestion is adding a "statistics" boolean flag. Any "statistics" field is automatically "private" and also hidden from the administrative list of all registrations. As we will only want to grab the final list once or twice (in random order maybe even separately for each field) there is probably little point in exposing it in the UI.