Add aural section headers for "Event Timeline", "Add Comment", and "Comment Preview"

Summary: See PHI871. Ref T13197. These sections are only divided visually and don't have textual headers. Add aural headers.

Test Plan: {F5875471}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13197

Differential Revision: https://secure.phabricator.com/D19654