provide race results in JSON form

[davewhiteland]

There are already paths for all races, and individual races... making them available as .json endpoints too which return JSON data of all the races and individual race results might not be too much work... and it would be helpful for the 5-RACELOG task.

Technically, each race for which we have a result does expose a "race file" JSON which contains both the results (and disqualifications) and events — it's the file that the race player uses to run the animation — but it's a bit tricky to identify what those URLs are (it's potentially custom for each race: you can see it in the admin by looking at the races)... so making this available as an explicit "get race results" endpoint seems useful.