Load hovercards on-demand

Summary: See discussion in D5563. This loads hovercards on-demand, and shows them once they load. Ref T1048.

Test Plan: Made a preview comment like `T1`, waved my mouse over it, got a hovercard shortly thereafter.

Reviewers: AnhNhan

Reviewed By: AnhNhan

CC: aran

Maniphest Tasks: T1048

Differential Revision: https://secure.phabricator.com/D5588