remarkup.css: Consistently zero top/bottom margins for first/last block-level children

Summary:
For block-level elements that have a margin-top or margin-bottom set
(generally to 12px), also reset the appropriate margin to 0 when
they're a first-child or last-child of their parents.

The change doesn't affect nested lists, their selector is more specific.

Test Plan:
Look at some comments or wiki documents that end with different
block elements, verify that the margins are pretty.

Reviewers: epriestley, Blessed Reviewers, chad

Reviewed By: Blessed Reviewers, chad

Subscribers: Korvin, epriestley

Projects: Remarkup

Maniphest Tasks: T6968

Differential Revision: https://secure.phabricator.com/D11382