Bug 21565: (follow-up) Make confirmation buttons more detailed

Bug 21565: (follow-up) Make confirmation buttons more detailed

This patch updates the confirmation buttons to be more specific about
what each one does. My concern was that it wasn't clear that "No" meant
"No receipt" and not "Cancel logging out."

The patch also updates the modal confirmation code so that the modal
body is removed if no message is passed for it.

To test, apply the patch and repeat the previous test plan. When you
click "Finish" you should have the options "Print receipt and end
session" and "End session."

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>