Update Settings/Config UI

Summary: Testing out a new 'nav' layout in Settings / Config. Spent a few days here and couldn't find much better overall.

Test Plan: View each page in Settings and in Config. Save some config options. Test mobile, desktop, tablet.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15659