Rework Mobile Header and Mobile Home.

Summary: Mostly finished, wanted to get it into your hands to play with. I need to remove some more dead CSS and figure out where we want to put profile/logout, but overall feels pretty good. Tested a bunch in iOS and other layouts.

Test Plan: Test Home, Maniphest, search bars, app menus.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin, AnhNhan

Maniphest Tasks: T2809

Differential Revision: https://secure.phabricator.com/D5407