Allow some content in a public universe to be private

This wouldn't actually be _too_ hard to do; the hardest part is just making sure users understand whether a particular piece of content is public or private. Currently, something is public if it's in a public universe _or_ specifically set public. We could change this so things are public only if it is set public _and_ in a public universe.

If we did this, we'd probably want to mark everything in a public universe as public (to keep current privacy settings) and then default newly-created content to the privacy setting of its universe, as well as give users an option to turn everything in a universe public or private (which might just replace universe privacy as a whole).

Feedback:

>Ability to create scenes within a universe that are hidden from others in that public universe.

>In regards to privacy settings, a feature wherein a character, item, or location can be hidden while within a public universe with the option to make it public at a certain point; this means that people can see a certain amount of information, and the creator can reveal information over time.

>Have private and public items/locations etc., so that you can have secret stuff that isn't just notes.

No files, tests, or entry points are named. Start by tracing the current rule that makes content public through its universe or explicit setting, then define the privacy behavior for existing and newly created content; done means the intended universe and content visibility rules are agreed and consistently implemented.

Written by the indexing model from the issue text.