Fix long thread titles in Conpherence

Summary: Fixes T3161 - Long Conpherence titles cause layout issues, picked a reasonable size to break it.

Test Plan: Create a long title, see the ellipses.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3161

Differential Revision: https://secure.phabricator.com/D5927