Use ellipis on long hovercard headers

Summary: This is a little cleaner than just cutting the title, like on Legalpad Hovercards

Test Plan: Test a Legalpad hovercard.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12722