Legalpad - style NOTE IMPORTANT WARNING remarkup slightly differently

Summary: round them there corners, to create more of a "bubble" effect in legalpad. Ref T3116.

Test Plan: see screenshot, which demonstrates new style works

Reviewers: epriestley

Reviewed By: epriestley

CC: chad, Korvin, epriestley, aran

Maniphest Tasks: T3116

Differential Revision: https://secure.phabricator.com/D8060