This change introduces new some functionality to the web API.

First, review requests can be set submitted, discarded, and reopened through the API. This is done through the /close/submitted, /close/discarded and /reopen and commands on the review request API tree.

Instead of handling the logic themselves, these commands call equivalent functions on the ReviewRequest object. This will be important for extentions down the road.

There's support now for retrieving the user's draft on a review request.

Posting updates to a review no longer requires supplying the body_top, body_bottom and shipit fields all at once. These fields can be supplied individually if desired.

And last but not least, this fixes a bug where we were generating a 500 when deleting the last comment on a review.

Unit tests pass, and these new APIs work in the UI rewrite.