Mention community fork in thank you message

What does this MR do and why?

Drops the message linking to "how to setup danger api token in forks" (which would always show regardless of whether the author already had it configured)... in favour of a responsive/reactive message encouraging the use of our community forks.

At the same time we took the opportunity to have a nice refactor 👌

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-with-a-dry-run on how to perform dry-runs.

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