More thorough validation for GitHub payloads

Summary:
Any GitHub payload must have:

• a X-Github-Event header
• a X-Github-Delivery header
• a payload, JSON deserializable as an object

A 400 Bad request error is sent if this is not the case.

Test Plan: Send a Phabricator request to /gate/GitHub/ProjectWithoutSecret

Reviewers: dereckson

Subscribers: alken-orin

Maniphest Tasks: T695

Differential Revision: http://devcentral.nasqueron.org/D262