[Redesign] Break words in headers

Summary: Ref T8099, Fixes T8678. Breaks overly long words in headers.

Test Plan: Make a long header on mobile.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8099, T8678

Differential Revision: https://secure.phabricator.com/D13453