Allow unlisted public artwork / A new gallery

[jvolker]

Sometimes it's handy to share an artwork URL with a friend without sharing it in the public stream – eg. for tests or feedback.

Maybe a way to implement this is to have a new gallery to replace the public stream that is a collection. This would also give us the chance to handpick artworks that are working. Users would then explicitly have to add their artwork to this gallery. Public artworks would not be listed by default.