Jupyterlab-hubshare

- Allow adding `other_link` to copy link to external site.
- Allow setting `file_path_func` to provide a more flexible way to configure path.

- Fixed no preview mode sharing path should include `/lab/tree/`

- Fixed decode file name

- Backward Compatibility

- Use b64encoded path otherwise jupyterhub user-redirect will try to encode string again and cause issue.
- simplify the share-url configuration

- support share and preview functions