Calendar event list items 'Attending:' field should only show users who have confirmed attendance

Summary: Fixes T8897

Test Plan: Open any list view of Calendar events, every event should only show "Attending: ..." with users who are attending event.

Reviewers: chad, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Maniphest Tasks: T8897

Differential Revision: https://secure.phabricator.com/D15779