Calendar event timeline feed should reflect invite changes better

Summary: Ref T7988, Calendar event timeline feed should better reflect invite changes.

Test Plan: Create or edit calendar event, invite or remove users, save, timeline feed should correctly reflect added/removed invitees.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7988

Differential Revision: https://secure.phabricator.com/D12625