Having the messeges embedded into onclick attributes was fragile because
it could break if the translated string contained special characters
such as single quotes.

Rather than trying to escape the special characters inside the onclick handler,
move the translatable strings into separate h-escaped data attributes and interpolate
the values in the (already existing) jQuery click handler rather than in the
inline onclick handler.

This makes the javascript code more consistent as well as fixes the problem
where the popup would break if translation included single quotes.