Don't parse JSON for unauthenticated requests

I notice the server is getting unauthenticated calls with random invalid JSON, resulting in JSON parsing errors.

I think it's better to first check that requests are authenticated before parsing their JSON payload.

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