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

src/__phutil_library_map__.php

@@ -5214,6 +5214,7 @@ phutil_register_library_map(array(
     'PhabricatorOAuthServerClient' => array(
       'PhabricatorOAuthServerDAO',
       'PhabricatorPolicyInterface',
+      'PhabricatorDestructibleInterface',
     ),
     'PhabricatorOAuthServerClientAuthorizationPHIDType' => 'PhabricatorPHIDType',
     'PhabricatorOAuthServerClientPHIDType' => 'PhabricatorPHIDType',
@@ -5350,6 +5351,7 @@ phutil_register_library_map(array(
     'PhabricatorPolicy' => array(
       'PhabricatorPolicyDAO',
       'PhabricatorPolicyInterface',
+      'PhabricatorDestructibleInterface',
     ),
     'PhabricatorPolicyApplication' => 'PhabricatorApplication',
     'PhabricatorPolicyAwareQuery' => 'PhabricatorOffsetPagedQuery',