Add processor for JiHu contributor to assign reviewer from gitlab-org

What does this MR do and why?

Adds a processor to allow the author of a JiHu merge request to ask @gitlab-bot to assign a gitlab-org member as a reviewer.

Action items

• When adding a new Processor, verify that it has been added to the DEFAULT_PROCESSORS in triage/triage/handler.rb in the appropriate order.
• If applicable, identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Week-in-review

Part of #173 (moved)

Closes #169 (closed)