Hovercards - make 'em work a bit better

Summary:
this diff makes hovercards load and draw but a single time as appropos; pre-diff we could hammer the server by moving the mouse a bunch before the initial load and the re-draw event was continuously firing.

also makes hovercards work inside whacked out divs like the Conpherence message panel. Fixes T2949.

Test Plan: played with conpherence and phriction, observing hovercards showing up like they should

Reviewers: epriestley, chad

Reviewed By: epriestley

CC: AnhNhan, aran, Korvin

Maniphest Tasks: T2949

Differential Revision: https://secure.phabricator.com/D5719