more layout tweaks, and aggressive data pruning
    
    these look unrelated, but actually they're not really. The two
    major changes here are to produce one table per release and
    have the tables float so they will line up side by side if you
    have sufficient screen width, and to prune all non-latest
    images from the data when writing it. They're related because
    both the layout and the pruning have to go through get_latests,
    and I had to rejig how that operates in order for it to work
    for the pruning.
    
    We can dial the pruning down later if we want the tool to do
    anything with non-latest images, but the data is not rare and
    precious, it's easy to reproduce, so there's no need to keep
    non-latest image data around 'just in case'; if the time comes
    that we need it, we can just re-seed. Nightlies are ephemeral,
    we don't need to be precious about storing data on them.
    Particularly since that's PDC's job anyhow.