backend: analyze storage use, and provide graphs
    
    New script #1: /usr/bin/copr-backend-analyze-results
        Run a 'du' command on the result storage directory and calculate
        several useful statistics.
    
    New script #2: /usr/bin/copr-backend-generate-graphs
        Go through the results provided by script #1, and generate the
        interactive graphs.
    
    New cron file: /etc/cron.weekly/copr-backend
        Execute script #1 and then script #2 to update the graphs.
    
    Closes: #1739