@luke @Tyagov 

In Jupyter notebooks users could enter the url of their `Base_executeJupyter` function, but [it now gets filled in automatically](https://lab.nexedi.com/nexedi/slapos/blob/2f2a5ad0/software/ipython_notebook/template/ERP5kernel.py.jinja#L10). This makes the `%erp5_url` magic redundant which is why we can remove it. This commit also removes all mentions of `%erp5_url` in the kernel output and `custom.js`.

/reviewed-on nexedi/slapos!185