Delete stories with a timestamp older than 30 days.

Such stories might have had their data truncated by previous delete-old-data logic.

sql/delete-old-data.sql

@@ -0,0 +1,5 @@
+-- delete data older than one month. It's taking about 18 days to fill a gig, and we have
+-- 3 gig volume currently. That gives us 54 days. But 30 days is plenty and this gives us margin
+-- in case the rate of growth of data increases.
+delete from dataset join stories using id
+where timestamp <= unixepoch()-30*24*60*60