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