Calendar event transaction should show a calendar icon in Feed

Summary: Fixes T8010, Calendar event transaction should show a calendar icon in Feed.

Test Plan: Update a Calendar event, check the Feed, transaction item should have a calendar icon in bottom left corner of the box.

Reviewers: epriestley, chad, Blessed Reviewers

Reviewed By: chad, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8010

Differential Revision: https://secure.phabricator.com/D12642