Fixed Issue #146: Leaderboard: allow navigation between items using keyboard arrow keys

[Issue #146](https://github.com/appirio-tech/arena-web/issues/146)

Updated to scroll the list when navigating using keyboard. To test, you can replace app\js\controllers\overviewLeaderboardCtrl.js with [this](https://www.dropbox.com/s/fi3o27ccok9ts2z/overviewLeaderboardCtrl.js?dl=1)