Update to new jupyterlab extension template

[florian-jaeger]

Is your feature request related to a problem? Please describe.
The original jupyterlab extension template cookiecutter is outdated.

Describe the solution you'd like
As suggested in the cookie cutter repo, we should us the new template called extension-template, which uses copier. This also enables us to automatically update the template using copier update in the future, if something changes in jupyterlab.