Fixed Differential quotes rendering. (phabricator)

Summary:
When quoting someone's text in differential, the text was not properly
rendered. Now the quoted text is rendered nicely and it's easy to locate.

Test Plan:
Disabled the inline cache, Checked that quotes in D211086 are rendered
correctly. Started writing a new comment starting with '>' and made
sure the new comment was rendered correctly while writing it.

Reviewed By: epriestley
Reviewers: epriestley
CC: jungejason, tuomaspelkonen, epriestley
Differential Revision: 138