Add icon for copy button

Add icon for copy button

Got the SVG from: https://icomoon.io/app

Compressed it with svgo (`npm install -g svgo`; see
https://github.com/svg/svgo)

Converted to data URI with: https://gist.github.com/puppybits/1565441

Did consider skipping the base64-encoding step (ie.
https://css-tricks.com/probably-dont-base64-svg/) but decided against
it. It won't work in old browsers, and isn't much of a space saving
anyway.