Handle watch event (star) on GitHub

Summary:
The watch event occurs when an user stars a repository.

The current star action on GitHub was the former watch,
and to avoid to break compatibility, the former name
remained.

Fixes T932.

Test Plan:

• Unit test with the payload from API doc
• Fire a payload received for Nasqueron 22:27:04 < Wearg> shaung starred nasqueron/docker-phabricator — https://github.com/shaung

Reviewers: dereckson

Maniphest Tasks: T932

Differential Revision: https://devcentral.nasqueron.org/D530