notebooks: add support for namespaces

Created by: novoselrok

Part of https://github.com/sourcegraph/sourcegraph/issues/30392

This PR adds support for notebook namespaces (user and org). The semantics of the namespaces along with the visibility option follow the example of search contexts and batch changes.

This is a large PR, but I did try to group the relevant changes by commits. So I would suggest to review commit by commit. This PR contains only a small amount of changes on the client, just enough to support the new namespaces parameter. The improved sharing capabilities will be added in a separate PR.