[RFE] Add a leaderboard page

[bmbouter]

We should have some leaderboards.

• There should be a leaderboard page which shows the usernames and which scenarios they have completed
• Also maybe the aggregate number of completed scenarios over time
• Possibly the top 10 users with the most completed scenarios

[bmbouter]

I made a basic leaderboard here: http://selinuxgame.org/leaderboard/