Enhance JSON output for rbt close.
Enhance JSON output for rbt close.

This updates the `rbt close --json` output to add the close description,
to rename `review_request` to `review_request_id`, and to add
`review_request_url`, for better reference to the review request.

It also changes `status` to `close_type`, since `status` is reserved and
was being overridden.

Testing Done:
Tested closing with and without a description. Saw the expected output.

Reviewed at https://reviews.reviewboard.org/r/12167/

reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools