Improve clarity around Remarkup blockquote display

Summary: This provides a little more separation/clarify around the blockquote design.

Test Plan:
Find a verbose passage from @epriestley, apply style.

{F147692}

{F147693}

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D8867