Fixed handling notebooks from shared folders

Shared folders have a different structure than shared files. Example:

https://www.dropbox.com/sh/mhviow274da2wly/CZKwRRcA0k/nested/furthernested/User%2520Interface.ipynb

This PR simply changes `www` to `dl` when the path starts with `s` or `sh` and adds a test for this type of URL.