Generalize ThanksContribution notifier and add custom conditions for different projects

Summary

Community Contribution thanks notifier logic will be moved to one class that handles overriding messages and conditions per project

Action items

• When adding a new Triager, verify that it has been added to the DEFAULT_TRIAGERS 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

Closes #98 (closed)