Update from CC 3.0 license to CC 4.0

Relates to https://github.com/github/training-web/issues/138 and https://github.com/github/training-kit/pull/128 : an effort to update the license to the latest and also label pages to make the CC licensing known and prominent.

Thanks to @idleberg for hosting the [CC licenses in Markdown format](https://github.com/idleberg/Creative-Commons-4.0-Markdown/blob/master/licenses/by.markdown).  :musical_score: "Everything is a remix"