Add triggers that automatically delete associated data when a user is deleted

When a user is deleted, Hue doesn't automatically delete the related records in applications like beeswax. This can leave dangling records, which will cause other problems. For example, if a user is deleted with the useradmin app then readded, rendering the beeswax query history for this user might reference the old user id, which will throw a DoesNotExist exception. We should use django signals to make sure these related documents are cleaned up.