Support Markdown in the Submitted/Discarded text fields.

We didn't have any proper support for Markdown text in the
Submitted/Discarded text fields before, due to lack of proper plumbing
in the API and in ReviewRequest.close().

Now it's possible to write Markdown and save it. It will render both in
the banner and in the change description box.

Reviewed at https://reviews.reviewboard.org/r/6409/
Improve close description support in the API.

The review request resource now has a 'close_description' field that
will be set to the close description text, if provided and if closed.
This makes it easier to see the status of a review request without
digging into other resources.

This also deprecates the 'description' field when closing a review
request through the API. The new argument is 'close_description', which
matches the returned value and doesn't conflict with the field.

Reviewed at https://reviews.reviewboard.org/r/6412/