frontend: fix the new copr logo
    
    Use just 'copr_logo.png', and move the old one to 'old_copr_logo.png'.
    First, the new path didn't work quite well (but I was lazy to debug
    staging why) and second we don't want to change the "flavor" API in
    copr-frontend.spec.
    
    Complements: d334b97a85ca8ba0be5e2d41f11344bbbbd4d92e