Improve the graph displayed in the stats tab of each projects
    
    Basically we're replacing d3js with chartjs which is much easier
    to work with and thus maintain in the long run.
    We're also using the new information about tickets to give a better
    view of the number of tickets opened or closed for each of the
    latest 53 weeks.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>