testcase-stats: fix bitmap sorting
    
    The bitmap images were incorrectly ordered since the recent
    wikitcms changes. I had changed testcase-stats' sorting of
    pages, but missed that it also sorts the list of all composes
    separately. That sort should still have more or less worked,
    so I'll have to look into why it's broken, but given that we
    now need wiki roundtrips for sorting, it pays to be smarter
    about it: let's sort the compose list properly (by creation
    date with wikitcms sort as a tie-breaker for composes from the
    same day), then re-use that sort for sorting the page lists.
    This not only fixes the bitmaps, it significantly cuts down on
    wiki trips, because we only have to do a trip once per non-
    nightly compose, rather than once per non-nightly *page* as
    before.