This adds JSON output to the rbt review command. The output contains

information relevant to the review, such as:

- The review ID.

- The review API URL.

- The review URL.

- The review status which can be draft, published, or discarded.

- The review request ID.

- The review request URL.
When a comment subcommand is used, the following comment details are also

included in the output:

- The comment type which can be diff_comment, file_attachment_comment,

  or general_comment.

- The comment ID.

- The comment API URL.
During testing, a regression in Review Board 5.0's API was discovered

which broke the reviews URI template on the Root List Resource. This breaks

the command in cases where a new review draft needs to be created. While the

API regression is being tracked and dealt with separately, this change allows

the command to work normally by accessing the reviews through the review

request resource instead of the root since we're fetching the review request

resource anyways in order to include its URL in the output.
An upcoming change will add unit tests for the rbt review command.
This also fixes a bug in the add diff comment subcommand where we add

the comment to the wrong file. The comment would always be added to the

last file in the diffset.

Manually tested the command and all of its subcommands.
Tested on RB5, RB6 and RB7.
Tested the case where a new review draft is created, and where one

  already exists.