R1:371fd12c1d5d

Flatten side nav

Summary: Debating removing the textures from the sidenavs, sending this around for comments.

Test Plan: Test homepage and various sidenavs, mobile layouts.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5948