Make "Copy" buttons actual buttons

This makes them keyboard-accessible. For mouse users, the only change
here is that the focus outline will remain on the button once clicked
(in future, this can be solved with :focus-ring, but for now this would
require a polyfill). the tooltip is explicitly hidden on `mouseleave`,
so even though the `<button>` retains focus after clicking, the tooltip
won't stay visible once mouse user moves away.
the mouse hover styles have explicitly not been make to also apply to
:focus, so as to minimise the visual impact for
mouse users (though the tooltip remains visible)
 - but due to the default outline and the custom tooltip, it
should be fairly clear when keyboard users set focus to a copy button
too.

docs/assets/scss/_clipboard-js.scss

@@ -23,6 +23,7 @@
   color: #818a91;
   cursor: pointer;
   background-color: transparent;
+  border: 0;
   border-radius: .25rem;
 
   &:hover {