frontend: adjust CounterStat.name length
We could just raise the limit from 127 to 400 because `Copr.name` can be
100, `User.name` can be 100, and chroot name can be 150, plus some
fluff that we add to the `counter_stat.name`. But `CoprDir.name` is
unlimited, so there might be problems. I'd rather use unlimited length
for `counter_stat.name`.