[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