Provide "diff-1up" and "diff-2up" classes and tweak some styles

Summary: Ref T2009. Remove forced min-width of 780px in 1-up mode, and tweak a few other things to look better.

Test Plan: Looks better on mobile.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D11974