Allow specifying ?force-text-type=html to Markdown-capable API resources.

Many of our resources support passing ?force-text-type=plain|markdown to
convert to either plain text or Markdown format. Now, these also support
"html" to render to HTML.

This is the first step to moving entirely to server-side rendering of
Markdown content.

Reviewed at https://reviews.reviewboard.org/r/6406/