Add severity override audit events

Overview

To keep security teams and administrators informed, and ensure they have visibility into the behavior of our vulnerability severity, create a new audit event. This event should be triggered when overriding vulnerability (or security finding) severity.

Requirements

• The audit event should be a project-level audit.
• Audit event message should include:
  • "Vulnerability severity was changed from X to Y"
• Audit event should include the vulnerability URL as its target.