Add automatic assignment for Threat Management Backend refinement

This randomly picks a Threat Management BE engineer from the hard-coded list to refine an issue.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Week-in-review