Correctly render Markdown in Changes field in emails.

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

Information

Review Board
master

Reviewers

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.