The existing 'status' field is done by directly changing the model, and

recording the change directly into the ChangeDescription. This worked okay,

except that trying to serialize change descriptions which included changes to

the 'status' field would explode.
This change adds a new ReviewRequestField (which is not shown in the UI), and

uses that to record the changes to the status, as well as serialize it in the

API. I've added a new unit test that verifies that we can serialize change

descriptions that manipulate the 'status' field.

Ran unit tests.
Loaded review requests and saw that there were no changes to the UI.