Django 1.10 introduced some changes to the way that middleware works.

Instead of having separate process_request and process_response

methods, middleware is now a callable which chains together. This

simplifies most cases to the point where most middleware can be

implemented as a nested function rather than a class. Opting into this

new behavior is done by changing the settings key from

MIDDLEWARE_CLASSES to MIDDLEWARE.
This change updates Review Board to use the new setting, and the

built-in middlewares to use the new pattern. As part of this, I needed

to upgrade django-cors-headers to a newer version. I've chosen the last

version that still supports Python 2.7.

Ran unit tests.
Smoke tested the Review Board UI.