Make it easier to copy solution snippets into the clipboard

Use ClipboardJS as discussed here: https://mooc-forums.inria.fr/moocsl/t/code-suggestion-for-the-next-session/3840/2

If we are lucky, this does not need too many manual changes in FUN-MOOC. Probably another custom js to add on our side.

An ideal solution (you may say I am a dreamer 😉) would be a single click to:
- add a cell into the exercise notebook with the code
- execute it
- scroll to the notebook and to the right cell directly