Enable hovercards for the "Task Graph" UI in Maniphest

Summary: See PHI118. Enables hovercards to support peeking at tags and other details if you, e.g., create numerous identical subtasks of each task.

Test Plan: {F5210816}

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18681