* add buttons to copy code block contents
Adds a clickable "copy" link in the top-right corner of each code block.
If available, uses the navigator.clipboard API. Falls back to selecting
the text and calling document.execCommand('copy') to copy text.
* hides copy button unless mouse is hovering over code block
* change text of copy button when text is copied
* add translation keys for copy button text `code_copy` and `code_copied`
* To disable use `Params.disableCodeCopy: true` in site config
Aditya Telange