Calendar event mentions should appear in event timeline.

Summary: Closes T7948, Calendar event mentions should appear in event timeline

Test Plan: Open Maniphest task, comment with a mention of an existing calendar event, go to event detail view, see timeline entry for the mention.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7948

Differential Revision: https://secure.phabricator.com/D12601