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
Maniphest Tasks: T992
Differential Revision: https://secure.phabricator.com/D10344