Add a border to additional property lists in a group.

Summary: When we stack property sections without a header, we still want a thin line. This adds a line on non-initial sections but not when headers exist. Fixes T4051

Test Plan: Tested Macro, File, Diffusion, a Whacked Profile Page and Maniphest. All lines look right

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4051

Differential Revision: https://secure.phabricator.com/D7492