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.