Make Remarkup Notes not have margin as first or last child

Summary: When a NOTE is at the top or bottom of a document, there is extra unwanted space.

Test Plan:
Write a NOTE as the top of a Phriction document

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11846