Prototype: context-sensitive sidebar

This is a prototype I'd like to flesh out over the weekend...

A more context-aware sidebar:

- click on a **Book** and the sidebar nav only shows the book (the workspace list _should_ only show content in the book)
- click on a **Folder** and the workspace list only shows the contents in the folder
- click on a **Book/Folder** and then click **Add**: The new content is added to the **Book/Folder** (also the types that can be added should be filtered)