[Redesign] Use Slabo font for headers, Remove Oswald

Summary: Couple of reasons here. Mainly, we're looking at Serif fonts for Phacility, so having headers tie back seems worth exploring. Also Oswals was a bit heavy, and in trying to find a replacement with similiar distinction, was unable to find anything suitable. Trying out Slabo here and have been pleasantly surprised how well the Serif/Sans-Serif combination works here, typographically.

Test Plan:
Test Objects, Documents, and Dashboards.

{F500207}

{F500208}

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D13279