Trigger hovercards on mousemove

Summary:
I often scroll with keyboard with cursor being somewhere on the screen.
Popping hovercards under the cursor is quite distracting.

Test Plan:
It works like I want in Firefox.
There's surprisingly no flickering even though I expected some.
It works like before in Chrome, perhaps it sends mousemove events anyway?

Reviewers: AnhNhan, epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5632