Leaderboard

This was originally #178, but now that a lot of pending stuff has been merged, the bases got screwed up, so it seemed like a good idea to just remake the PR, especially since @mythmon basically recommended I rewrite most of the PR anyway. And I did!

This adds a management command for updating the leaderboard that pulls in the data using Django querysets and some bulk insertion, as opposed to raw SQL. It also adds a shell script that we can use for the nightly update, so that the leaderboard management command isn't run until after new clicks are retrieved from GA.

Also also this does some partial integration on the leaderboard page. It's not complete, but it's a start! :D