Maybe possibly really fix :last-child in phabricator-remarkup

Summary: This rule should correctly target the last direct descendent child of .phabricator-remarkup, and not grandchildren. Fixes T9833

Test Plan: Test commenting, documents, descriptions, and other remarkup areas. All spacing seems consistent.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T9833

Differential Revision: https://secure.phabricator.com/D14548