Allow to pick a random coach from a group

What does this MR do and why?

This is a prerequisite for !1454 (merged) and !1453 (merged).

Expected impact & dry-runs

The current behavior shouldn't change.

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 =>
  • Engineering week-in-review