kcms/feedback: Only add audit locations to the model that exist
This prevents audit locations that don't appear on disk from showing up as clickable links in the KCM, which produces a bad UX whereby you can try to open the nonexistent location and then you get an ugly error message instead. So let's just not even show them if they don't exist
BUG: 445959 FIXED-IN: 5.24
cc @apol