Update Settings to use TwoColumn fixed layout

Summary: Simplifies the page, adds base support for PHUITwoColumn fixed from Instances (which I'll delete css there).

Test Plan:
click on every settings page, UI seems in tact, check mobile, desktop, mobile menus.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D18436