Product description markdown parsing

We want product description to be markdown, so this parses it
for HTML output, however not for JSON API, that we will do.

Signed-off-by: Zeljko <zeljko@zwr.fi>
JSON serializer renders

Note that I do not like that serializers require explicite
rendering. Check the controller and monitor mentioned
SO question for how to revert to empty Show methow.

Signed-off-by: Zeljko <zeljko@zwr.fi>