Add priority to incidents

Proposal

Currently, incidents have a built-in severity field.

At GitLab-SIRT, we make use of both severity and priority to denote the criticality and urgency of an incident. Certain incidents are high in severity but once contained, the priority for handling/investigating that incident decreases. Adding an extra field for priority will help us gain insights on the current status of incidents.

We are currently making use of a priority1 label.