Add features needed for CEM site

Specifically:

* Add a `processed` property to reviews which defaults to false, but can be set to true via an API call.
* Add a `meta` property to submissions which can store arbitrary client-specific JSON metadata about a submission. Currently, this can only be set upon the initial creation of a submission, because it's all we need for CEM, but in the future it can be enhanced to support changing via API calls.

The updated documentation can temporarily be viewed at http://cem-aestimia-docs.toolness.org/.