[Redesign] Move headers to Oswald font

Summary: Ref T8099, I want to live with this for a few weeks. I've grow to like that it gives more distinction to ObjectBox Headers and fits very nicely as headers in DocumentView. Even if we don't use them for all headers, Documents are much nicer to me at least with a more visually impactful font (Source Sans doesn't make a great header).

Test Plan:
Review dashboards, objects, Diviner, Phriction, Legalpad, and misc other pages.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D13127