Try using a less-rich but less-risky thread title in Conpherence handles

Summary: Ref T8478. This is easier than I thought, let's see if it's conspicious?

Test Plan:

• Embedded {Z2} and saw "Private Correspondence".
• Still saw normal stuff with useful participant lists/titles in all main UIs.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8478

Differential Revision: https://secure.phabricator.com/D13220