coreos-download: add summary for packages and pkgdiffs
    
    Adds a section at each release note that summarizes number of packages
    that ships in the release and number of pkgdiffs between previous release.
    Appends a button for each pkg summary that expands/collapses the corresponding
    pkglist/pkgdiff section on the release note.
    
    Signed-off-by: Allen Bai <abai@redhat.com>