CiviMail 'click success rate': base on opens not intended recipients

See https://forum.civicrm.org/index.php?topic=37015.0:

It looks like the 'success rate' for individual links in mailings is calculated based on the number of intended recipients. ie:

unique clicks for link / intended recipients * 100 = success rate for link

This raises the philosophical question of whether a link can be considered a 'failure' if the email wasn't even opened. We are considering changing the formula for success rate to:

unique clicks for link / unique tracked opens * 100 = success rate for link

Then you're tracking whether a link is successful based on how many people have seen it, which makes a little more sense to us.