Ping JiHu Slack when changes detected in .eslintrc.yml

What does this MR do and why?

Ping JiHu Slack when changes detected in .eslintrc.yml

When a merge request is merged.

This is based on !2197 (merged)

Expected impact & dry-runs

This is tested by merging godfat-gitlab/test!3 (merged) and that project has webhook set pointing to my local machine, which set JH_SLACK_WEBHOOK_URL to the value we normally use for Slack notifications so I am able to see. It worked (internal):

Of course, I commented the check event.from_gitlab_org_gitlab? locally so it worked for my testing project.

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

Closes #1216 (closed)