jupyter: Simplify ERP5kernel maintenance

Instead of templating python code, make it static and parameterize it with parameters.

kernel.json can pass default ERP5 url, which is (dirty) read by ERP5kernel. Additionally ERP5kernel.py is just linked into kernel directory, which simplifies debugging.

Note: This commit is still dirty, but it prepares ERP5kernel.py to be packaged, thus there is no reason to make more changes and cleanup.