Add copy to clipboard button to code blocks

What

• Closes https://gitlab.com/gitlab-com/gitlab-docs/issues/56
• For epic gitlab-com&4 (closed)
• Add copy to clipboard button to code blocks:

• Implement ClipboardJS
• Make it work
• Style it
• Fix style
• Add tooltip saying "copied"
• Add button title saying "click to copy"

Review Apps:

• http://copy-button.178.62.207.141.xip.io/ee/ci/yaml/
• http://copy-button.178.62.207.141.xip.io/ee/ci/examples/devops_and_game_dev_with_gitlab_ci_cd/
• Scroll-x: http://copy-button.178.62.207.141.xip.io/ee/user/project/integrations/prometheus_library/kubernetes.html