reactive: Add a new `request_review` command

What does this MR do and why?

This adds a new request_review command that post a message to the #mr-coaching Slack channel.

Expected impact & dry-runs

The goal is to gauge the usage of this command, before potentially expanding it to post a Danger review message when requested.

Action items

• (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 => @cbacharakis
  • Relevant Slack channels => #mr-coaching, #core
  • Engineering week-in-review

Closes #767 (closed).