Fix HTML email section breaks

Summary: Fixes T6481. Provides better section handling.

Test Plan:
Test an email with multiple sections, multiple mail platforms.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6481

Differential Revision: https://secure.phabricator.com/D10811