[Redesign] Swap Slabo for Roboto Slab
Summary: Ref T8099, Moving to Roboto Slab for Document Headers. It's a little less serif-y and fits well with Lato. Also took a pass at cleaning up edge cases in Documents, Diviner, Phriction, Legalpad, and Welcome Screen.
Test Plan: Test Phriction, Diviner, Legalpad, Welcome Screen
Reviewers: btrahan, epriestley
Reviewed By: epriestley
Subscribers: epriestley, Korvin
Maniphest Tasks: T8099
Differential Revision: https://secure.phabricator.com/D13439