Move hovercard z-index up

Summary: Fixes T2912. I put this above dialogs but below tooltips, which I think is the right place.

Test Plan: {F39797}

Reviewers: chad, AnhNhan, edward

Reviewed By: chad

CC: aran

Maniphest Tasks: T2912

Differential Revision: https://secure.phabricator.com/D5629