Threat Insights FCL - Create a team-based PTO handoff policy

This issue is part of our Threat Insights Frontend Feature Change Lock.

Create a team-based PTO handoff policy where non-draft MRs are handed off to another team member.

Key results:

1. ✅ Active (non-draft) MRs should be reassigned when a team member goes on PTO.
2. ✅ Update handbook with handoff-steps including re-assign MR.
3. How does staging verification issue determine who to ping? Author, assignee? If author, we need to get updated to include assignee.