Fix customer contribution notifier to not check for community author on merge event

Summary

The original code was checking that the merge event was from the community author. We only need to check that the MR was authored by a customer author. We can remove the check against the merge even author because this will be a GitLab team member.

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 #99