Change python intro to use files

Changes to the Python Introduction chapter to use files when things get more complex.

This reshuffles the code editor chapter to between the Python installation and Python introduction chapters.

Reading through the tutorial, it felt like the interpreter was a good choice up until conditional loops and functions were introduced, so I didn't rewrite the whole thing. I did however add a small section on getting the code editor open and why we are switching to it at that point (see line 396 and onwards in python_introduction/README.md)

Closes #207 and #190

:gift: 
My general guideline is: single quotes for machine-readable strings and double quotes for human readable ones.

It might be nice to be more consistent about this through the tutorial but I think that's out of scope for this PR.

Also note that we do explain the two syntax at the beginning of this chapter and in my exprience, I haven't seen much confusion about this in practice.