Add contributor leaderboard #240
Description
Add a page /leaderboard that shows the users with the most contributions.
Split up contribution details by type: spatial metadata or temporal metadata, using appropriate icons.
Users need to opt-in in their settings and define a username there to be displayed on the leaderboard. Use a random username generator API to fill in a default value when the user enables the dashboard.
Statistics are always collected internally even if the user does not allow to include them in the leaderboard.
Update the provenance log and the user database table to enable this.
Add 5 tiers of contributors to group the users with a logarithmic scale to "earn" the contributor level.
See https://www.historyhit.com/most-important-explorers-of-the-world/ for a list of explorers.