allow private code submissions

[mschrimpf]

There are quite a few users who prefer to not make their submitted model code public, e.g. because it's under active development. I would like to support that use case in the future too where users can e.g. submit a model before publication, include the results in their paper, and later opt to make it public. In other words, it should be possible to receive a score for a private model without making any of the code public.

One way to implement this (suggested by @kvfairchild) is to manage a separate private repository with private registries for internal scoring use only, that we can internally still score on and make the scores visible to the user (and potentially publicly if selected).