Fix break in Plotting in Python Using Matplotlib and some CSS fixes

[jmuzsik]

👋 Hi

This is a simple fix. I had an error when visiting https://nbviewer.jupyter.org/github/qutip/qutip-notebooks/blob/master/python/Matplotlib_Plotting.ipynb. As, the code examples were hidden. The reason stated in the console was:

Mixed Content: The page at 'https://nbviewer.jupyter.org/github/qutip/qutip-notebooks/blob/master/python/Matplotlib_Plotting.ipynb was loaded over HTTPS, but requested an insecure stylesheet 'http://fonts.googleapis.com/css?family=Source+Sans+Pro'. This request has been blocked; the content must be served over HTTPS.`

So, I replaced the http://fonts.googleapis.com with https://fonts.googleapis.com and that should do the trick (it cannot be checked on localhost). I also ran an automatic linter on the css code and deleted a couple errors.

[nathanshammah]

Hi @jmuzsik, thank you for the feature addition. I am no html expert, but in that notebook it says:
"The raw code for this IPython notebook is by default hidden for easier reading. To toggle on/off the raw code, click here."
It seems to work fine for me, on Chrome, Safari and Firefox.

[nathanshammah]

Would only switching to https be fine?