[Redesign] Touch up sidenav UI

Summary: Ref T8099, lots of little tweaks, mostly to icon nav.

Test Plan: Test side navigation elements on home, workboards, conpherence, and maniphest

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D13456