Tweak panel-filter shadow.

Summary: Adds a consistent shadown under crumbs, tweaks filter-panel to look a bit crisper

Test Plan: Tested crumbs with filter, with nothing, and with a header.

Reviewers: epriestley, btrahan

Reviewed By: btrahan

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5353