Add "challenge" leaderboard page.

[Meowse]

Site should have a page that displays the current challenge leaderboard (for this week's and last week's challenges). Makes it easy for users to keep tabs on the challenge results, and provides another channel for announcing the new challenge.

Implies some means of specifying the challenge stat for each week (probably a simple data table would suffice, since challenges could be set up for an arbitrary number of weeks in advance).

[johnluetke]

I like this.

I could see a Challenges table that defines the stat, start_date, end_date, and threshold so that such a page can be created automatically, and provide a historical view of challenge results.