Make the 404 page device friendly.

Summary: Fixes T5686.

Test Plan: Viewed in iOS simulator and Chrome with various viewport sizes - looked good!

Reviewers: chad, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5686

Differential Revision: https://secure.phabricator.com/D10082