Switch over to the new Markdown support in Djblets.
Review Request #7317 — Created May 18, 2015 and submitted
Much of the custom Markdown processing code in Review Board has been moved over to Djblets. We're now using this centralized code instead, wrapping it where necessarily to provide backwards-compatibilty (with deprecation warnings) for extensions. This is less code to maintain in Review Board.
Unit tests pass.
Tested Markdown rendering, e-mail previews, and Markdown file attachments.