Add copy-to-clipboard buttons to code blocks

Though we generally type all code while teaching, I would have find it useful recently, while preparing to teach a lesson, to be able to copy a code block easily. I can select the block and then copy and paste it. It would however be easier, if there was a button that would cause the block’s contents to be copied to the clipboard (Similar to the buttons in the Clone-Dialog of GitHub next to the repository’s URL).