Add cron job that generates flow statistics

reverting the revert from https://gerrit.wikimedia.org/r/#/c/171465/
because the limn-flow-data repository is ready to run jobs now, after
https://gerrit.wikimedia.org/r/#/c/176789/

Change-Id: I4d64ea87e7c1ef35e2bbad24fe088a9c385ffa3f