Add Maniphest Hovercard event listener

Summary:
Refs T1048
badassery

Test Plan: Used in a future diff. Verified correct appearance of hovercard

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1048

Differential Revision: https://secure.phabricator.com/D5544