Format review app report slack message

The following discussion from !29 (closed) should be addressed:

• @rymai started a discussion: (+1 comment)

  @kwiebers This fixes the report to make it actually work. I didn't manage to make the Markdown display as expected in Slack, I think we'd need to extract the payload to a separate file, similarly to how we do in https://gitlab.com/gitlab-org/quality/toolbox/-/blob/master/reports/quarantine_report.json.erb.

  /cc @caalberts