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: 
In my experience, nobody knows what a colon is (especially not non-native English speakers).

I think it would work better to explain that `if` and `for` behave similarly in that they need to have an indented block after them.