Add missing `key` to flatpages ordering
Add missing `key` to flatpages ordering

What has been removed in 4067b94c212d1a41624b729a1aa114e6dca64d05 was
actually not redundant.  `cmp=compare` made use of the _custom_
`compare` function defined above.  Anyway, this could be replaced using
a more beautiful `key=attrgetter('rank')`.

lukasjuhrich/sipa,lukasjuhrich/sipa,agdsn/sipa,fgrsnau/sipa,fgrsnau/sipa,MarauderXtreme/sipa,fgrsnau/sipa,lukasjuhrich/sipa,agdsn/sipa,MarauderXtreme/sipa,agdsn/sipa,MarauderXtreme/sipa,agdsn/sipa,lukasjuhrich/sipa