htdocs: Add clipboard.js

htdocs: Add clipboard.js

We provide our own, much smaller implementation of this currently.
However, we want to be able to implement slightly different variants of
this elsewhere and using an existing library helps avoid reinventing the
wheel and lets us use already battle-tested code.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>