[DEV-2153] Refashion "People" page into a leaderboard.

@mwhansen cr?

The leaderboard template is mostly a copy-paste of the user list page template from Askbot, with some pieces removed and a few pieces that were previously in macros inlined and modified.
I'd rather this be something like

1. Create a queryset representing all of the users which would appear above this one in the leaderboard (filter user that have greater reputation, or equal reputation and last_seen sooner)
2. Return the number of users in that queryset + 1