[Redesign] Normalize nav spacing

Summary: Ref T8099. Not sure if this was accidentally removed, but overflow: hidden saves the day once again. Fixes an issue where different pages had navs starting at different spots based on crumbs or object boxes.

Test Plan: Test Projects, People, Dashboards, lots of other navs. All start at 8px now.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D13047