Private sheets

As a content creator, I would like the ability to make sheets that are not exposed via Helix.

**Criteria of acceptance**
1. When previewing or publishing, a sheet I deem private is not available publicly.

**High level thoughts**
I see two decent paths:

1. Stop automatically exposing all sheets and use the `shared-` prefix like Excel.
2. Have an opt-in for private data and use something like a `private-` prefix.

Each option has downsides:
Number one requires more training for net-new users... "Why is my sheet not visible?"
Number two differs from other implementations so more difficult for existing SharePoint customers.