Post frontend incidents to the frontend channel

What does this MR do and why?

Closes Identify and assign fronted alerts to related g... (gitlab-org/quality/engineering-productivity/team#253 - closed)

Redirect frontend incidents to the #frontend Slack channel, unless the incident has other failures that can be attributed to a group.

For the first iteration, we will post master incidents with jest test failures to the frontend channel to ensure it's triaged and resolved quickly. If we receive any negative feedback about this notification being too noisy for that channel, we can revert it.

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-policies-with-a-dry-run on how to perform dry-runs for new policies.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/doc/reactive/best_practices.md#use-the-sandbox-to-test-new-processors on how to make sure a new processor can be tested.

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