Create service `nbviewer`#

Configuring the notebook viewer as a JupyterHub service has the advantage that only users who have previously logged on to JupyterHub can call up the nbviewer instance. This can be used to protect access to notebooks as a JupyterHub service in /srv/jupyter/jupyter-tutorial/jupyterhub_config.py:
```
c.JupyterHub.services = [
    {
        'name': 'nbviewer',
        'url': 'http://127.0.0.1:9000',
        'cwd': '/srv/jupyterhub/nbviewer-repo',
        'command': ['/srv/jupyter/.local/share/virtualenvs/jupyter-tutorial--q5BvmfG/bin/python', '-m', 'nbviewer']
    }
]
```
name
The path name under which the notebook viewer can be reached: /services/NAME

url
Protocol, address and port used by nbviewer

cwd
The path to the nbviewer repository

command
Command to start nbviewer