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