Statistics and visualizations per challenge

[slashnick]

Difficulty, success, challenge popularity, completion speed, etc.

[slashnick]

Some of this stuff was specific to OverTheWire (like completion speed). I think solve counts (and aggregate statistics about solve counts) should be the main metric we look at for challenges.

Time of first solve is interesting, but with a small CTF it tells us more about teams' strategies (eg. do the lowest point challenges first) than about the challenge itself.