Add leaderboard support.

* Two leaderboards created per org. ALL_TIME and THIRTY_DAY
* Cron job to update leaderboard every 24 hours. Modification info in leaderboard indicates when the leaderboard was refreshed. The UI should display it.
* API to fetch the leaderboard

    /api/{org_key}/leaderboard?type={leaderboard_type}
        
    leaderboard_type=ALL_TIME | THIRTY_DAY

* Convenience: "Compute organization leaderboard" button to launch map-reduce in /bootstrap
  Note that this is automatically done when "Bootstrap test resources" is clicked.