Render markdown fields for HTML emails.
This change adds some slightly questionable code to render markdown to HTML for

the HTML emails. Annoyingly, python markdown doesn't interpret the input quite

the same as marked.js, so some things don't render the same (this is mostly

edge cases, particularly with the treatment of things like whether lists

require a blank line before them or not).
This includes a simple extension for python-markdown that will go through the

resulting element tree and add inline styles. These styles are similar to what

we get from reviews.less, but somewhat simplified.

Viewed the html email previews for review requests, reviews, and replies with

markdown in the review request fields, review body_top/bottom, and comments.

Saw all of the fields rendered to markdown, instead of seeing all the control

and escape characters.