Add a box around the pager in Diffusion
Summary: Fixes T9392, adds some sweet sweet margin to the pager.
Test Plan: See pager with new padding, test different pages, breakpoints.
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin
Maniphest Tasks: T9392
Differential Revision: https://secure.phabricator.com/D14098