Add slack channel overrides for group notifications

Context

Closes gitlab-org/quality/engineering-productivity/team#334 (closed)

What does this MR do and why?

Add slack channel overrides

We don't always want to notify of incidents in the group slack channel.

This MR adds an override mechanism, so that we can override slack channels for notifying groups.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (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