[WIP] Add status-update command to rbtools.
Review Request #9343 - Created Nov. 1, 2017 and updated
Review Board 3.0 introduced new support for "status updates" which are
used to indicate if a review request is undergoing some automated checks
of some kind (for instance, automated code review or pending builds).
These can have a state (such as "pending", "error", "success", and
others), a summary, optional link, and a review (when there are errors).
This adds a new command for working with status updates, helping to
create/update the state of a status update. This command can be run with