Comment 4 for bug 1648921

When I looked earlier, I couldn't easily backport this to 16.04 LTS.

After a few months, I believe the tracker sandbox still causes regressions since it blocks stuff that used to be allowed. The new tracker (with sandbox) will be shipped in 17.04 and Debian stretch, but I think this update is not worth doing for 17.04 given that 17.04 is already halfway through it short life and given that we are unlikely to be able to fix all regressions it introduces.