Fix error in friends page.
Change leaderboard sort to be GET instead of separate URL.
Change ForeignKey(User) to ForeignKey(Sleeper) for convenience and efficiency.
Revert "Change ForeignKey(User) to ForeignKey(Sleeper) for convenience and efficiency."

This reverts commit a9f2f6885cc0ad9d4d92906093d86bb7ce993040.

Using the proxy model in the ForeignKey should work, but South chokes on
the migration (cf. http://south.aeracode.org/ticket/736), so reverting
it.
Merge branch 'master' into pretties