add jupyter notebook integration / css / agree on style

@jonas-braun @ellenkoenig
Using Jupyter notebooks with Jekyll is fairly straightforward, according to www.linode.com, if you simply export your JN as MD. That's good enough for me. But it might require some CSS tweakage, and there there's the question of how/where graphics and images are stored. Also, the style guide might need to be updated to account for JN formatting issues – such as output, which can be looooonng.

[jonas-braun]

for now, lets use this schema:
export jupyter-notebook as markdown and paste it into the jekyll source files. input blocks look like this:

countries = list(set(user_countries.values())) 
print(len(countries)) 

(three backticks and the language python)
and output blocks look like this

162

(no backticks, simply indented using white spaces)

lets make a nice stylesheet later and keep the black background to stick with the OTS stylesheet.

[jonas-braun]

I updated the stylesheet to give us a new look for juypter-style code blocks. See loop example in https://opentechschool.github.io/python-data-intro/core/recap.html
An html tag around the code causes it to be formatted as input and output of a jupyter script.

Looks good! We might want to ditch the border surrounding the cell. It cuts through the "show" button.