Use a single newline between HTML mail sections

Summary: Ref T992. This makes HTML mail layout more consistent with text mail layout and fixes my greatest annoyance with it.

Test Plan: Used bin/mail list-outbound --id <id> --dump-html to view mail in Safari, saw it have a normal amount of whitespace between sections.

Reviewers: btrahan, talshiri, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T992

Differential Revision: https://secure.phabricator.com/D10344