Make Jupyter config initialize block code instead of a string

[jayqi]

From @pjbull (source):

let's see if there's a good way to store this block as Python code so it gets highlighting in source control and could be easier to test directly if we need to eventually. I think that we could either define the function and use inspect.getsource to get this code after the function is imported, or maybe just have it in a .py that we read instead of importing.

I personally like the idea of doing it in a function. Then we can pass in c as an argument, which would satisfy linting, and also make it possible to do some basic compatibility testing against Jupyter in the unit tests.