Add GitHub Ribbon to our lessons

Backport of 9a19e91 from swcarpentry/site.

All credits to Abigail Cabunoc <abigail.cabunoc@oicr.on.ca>
for the awesome work with CSS.
Improve CSS for GitHub ribbon

- For small screen, preserve margin.
- Use css-beautify
Merge pull request #447 from r-gaia-cs/github

Add GitHub Ribbon to our lessons