This makes it so that posts (both topics and comments) can no longer be
voted on after they're over 30 days old. An hourly cronjob makes this
"official" by updating a flag on the post indicating that voting is
closed. The daily clean_private_data script then deletes all individual
vote records for posts with closed voting, and the triggers on the
voting tables have been updated to not decrement the vote totals when
these deletions happen.
The net result of this is that Tildes only stores users' votes for a
maximum of 30 days, removing a lot of sensitive/private data that builds
up over the long term.