Correctly render Markdown in Changes field in emails.

Review Request #7833 — Created Dec. 27, 2015 and submitted — Latest diff uploaded


Review Board


Change description text in review request emails previously did not
correctly render Markdown formatted text, even if Markdown was
enabled.  Now, the rich_text field is leveraged for enabling
Markdown, similar to how it is done when rendering other fields.
Examined both text and HTML emails with all combinations of 
Markdown enabled/disabled and change description text which
both did and did not contain Markdown formatting.