Move all CSS "z-index" rules to "z-index.css", fix Differential z-index problem

Summary: We have enough z-index rules that they're fairly hard to visualize with "git grep". Consolidate them. Then fix T2253 (missing z-index on left menu background).

Test Plan: Made a Differential window really narrow, then scrolled it horizontally.

Reviewers: btrahan, chad, ender

Reviewed By: chad

CC: aran

Maniphest Tasks: T2253

Differential Revision: https://secure.phabricator.com/D4302