Calendar events should actually have an icon now.

Summary: Ref T7936, Calendar events should actually have an icon now.

Test Plan: Edit event, edit icon, save, observe transaction feed.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7936

Differential Revision: https://secure.phabricator.com/D12934