Add rank assessment breakdown for beta kata

[hobovsky]

From kazk:

For this feature, I'll add /api/v1/code-challenges/:id/assessed-ranks that responds with JSON like

type Response = {
  data: Count[], // ordered decending by count
};
type Count = {
  rank: number,
  count: number,
  users?: string[], // user ids, only if requested by mods
};

{
  "data": [
    {"rank": -7, "count": 4},
    {"rank": -6, "count": 2},
    {"rank": -1, "count": 1}
  ]
}