Fix showing the number of PR involved
    
    Since we used to have all the PRs showing on the page, we were simply
    counting with JS the number of rows/PRs shown. Now that we paginate we
    can no longer do this and instead we need to rely on the information we
    retrieve from the database.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>