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