Add styles for WARNING and IMPORTANT

Summary: Ref T3116. I did not update the remarkup doc (yet) as I think this syntax should stay buried until the bubbler looks right

Test Plan: modified a legalpad document and verified BUBBLE: showed up and looked okay to my pitiful design skillz

Reviewers: epriestley, chad

Reviewed By: epriestley

CC: chad, Korvin, epriestley, aran

Maniphest Tasks: T3116

Differential Revision: https://secure.phabricator.com/D8053