Simplify/clean-up leaders code a bit.

Avoid the grouping by `Score.key`, as we already filter by a single key.
Add a new 7-day leaderboard showing new unique cell/wifi discoveries.

Currently limited to Top-20, until we add paging, refs #24.
Merge pull request #181 from hannosch/24-added-top20-weekly-leaderboard

Added a simple top-20 seven-day leaderboard