Calendar events should support comments

Summary: Closes T7956, Calendar events should support comments.

Test Plan: Open event, add comment, save, comment should appear in timeline.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7956

Differential Revision: https://secure.phabricator.com/D12640