Add Daily Challenge rankings by total participation.

[GabuTheDev]

I was wondering who has the longest streak so far, had no data to look into, so I though it would be nice to have a leaderboard page with this. Could be similar to the Kudosu one, showing total participation, best daily streak and best weekly streak as columns.

[peppy]

We intentionally didn't do this because we didn't want to make it a competitive system.