Support use of `JupyterHub.allow_named_servers`

### Proposed change

If I understand correctly it is currently not possible to create named servers with systemdspawner.

If you set

```python
c.JupyterHub.allow_named_servers = True
```

Creating a new named server will kill the original server. I *think* this is because the `unit_name_template does not expand the name:

```python
c.SystemdSpawner.unit_name_template = f'jupyter-{USERNAME}-singleuser'
```

thus the "named server" has the same name as the default server and this the default server is killed...

### Alternative options
- Not using named servers.
- Issue a warning when `c.JupyterHub.allow_named_servers = True`

### (Optional): Suggest a solution
make `c.SystemdSpawner.unit_name_template` expand the (optional) name of the server