Use white "+" icon for homepage tile hoverstate

Summary: The easiest approach here is proably to provide a more specific rule in the sheet CSS. This saves us from having to write any JS, notably.

Test Plan: Hovered over "+" on homepage.

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D4579