Handle issue_comment event on GitHub

Summary:
Reference: https://developer.github.com/v3/activity/events/types/#issuecommentevent

Fixes T844.

Test Plan:
Unit tests have been added for this new event,
with sample payload from GiHub API documentation.

Reviewers: dereckson

Subscribers: alken-orin

Maniphest Tasks: T844

Differential Revision: https://devcentral.nasqueron.org/D686