Add background to haunted differential comment panel

Summary:
Warning boxes in a haunted differential comment panel were showing up
without a background (so the contents below were showing through).
This adds the background.

Test Plan:
viewed haunted differential comment panel on a diff with lint errors
and selected "accept revision". Confirmed that the diff and sidebar did
not show through underneath the red warning box.

Reviewers: epriestley, chad

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4154