Add prio/p1 by default for bug instead of notif/triage

[igarashitm]

Now it's a bit annoying to remove notif/triage everytime. Instead I'd like to have prio/p1 applied when cat/bug is applied but there's no prio/* label.

[rhuss]

Another idea is to add "notif/triage" only if not any label is given during creation and automatically removed as soon as a label has been added. So no manual removal is required anymore.