AphrontFilterView should have 8px spacing on tablet

Summary: Adds a tablet breakpoint CSS for better filter alignment on tablet.

Test Plan: Review desktop, tablet, and mobile breakpoints on an ApplicationSearch

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12332