backend: fixup systemd unit ordering
    
    The 'copr-backend' oneshot script needs to be run before we start taking
    build jobs from frontend in 'copr-backend-build'.
    
    While we are on it, declare that all our services need to have
    'copr-backend-log' service started first (always was truth).
    
    Also drop wrong Before=copr-backend.service statements where we don't
    care about ordering.
    
    Fixes: #1113
    Merges: #1115