[DO NOT MERGE] Leaderboard

Updates the `collect_ga_data` management command to be the `nightly_update` and adds leaderboard generation to the nightly update. No need to review the details of collecting the GA data, that's happening in another PR, just focus on the new command's structure and the leaderboard update parts.

[Also has a commit for adding Gravatars that was copy-pasted from Captain, which in turn was copy-pasted from Flicks, which in turn was copy-pasted from the old Affiliates site when I first wrote it.](https://www.youtube.com/watch?v=HwSKkKrUzUk)