Move settings from server to lab

The server extension can be configured by adding
```python
c.ThreddsConfig.maxtasks = 100
c.ThreddsConfig.timeout = 60
```
to `~/.jupyter/jupyter_notebook_config.py`.

For a lab extension it would make more sense if the settings are configurable from the lab advanced settings editor. See https://jupyterlab.readthedocs.io/en/stable/developer/extension_dev.html#extension-settings for more info.