Fix JSON lb endpoint yet again.

It's roughly efficient-ish now (but includes a lot less info).
Redo how /leaderboard works to make it faster.

- Only pull info we need, and do so in only one query.
- Update template to reflect the new data structures.
Wrap lb json endpoint in a "leaderboard" field.
slightly comment how this works.
Merge pull request #173 from fedora-infra/lb_json

More leaderboard json stuff. Maybe it works this time.