Add support and very basic styling for quotes in Phabricator

Summary: Ref T4119. Adds the block rule and makes a faint effort at CSS.

Test Plan: See D8953 for a screenshot.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4119

Differential Revision: https://secure.phabricator.com/D8955