Calendar event detail view should show "Unnamed Event" as title for events with no title

Summary: Closes T8048, Calendar event detail view should show "Unnamed Event" as title for events with no title

Test Plan: Open an old event created before titles were required, event detail view should display title as "Unnamed Event" instead of a blank title

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8048

Differential Revision: https://secure.phabricator.com/D12736