Lazily load "person" for leaderboard for a speed boost.

i.e., solve one occurance of the N+1 problem.

```
$ time curl http://localhost:8000/leaderboard/json -so /dev/null

real	0m7.287s
user	0m0.000s
sys	0m0.003s

$ git checkout leaderboard_slowness
Switched to branch 'leaderboard_slowness'

$ time curl http://localhost:8000/leaderboard/json -so /dev/null

real	0m1.317s
user	0m0.003s
sys	0m0.002s
```