Review Board 2.5 introduced support for using API tokens for

authentication, but this was limited to Review Board. This change ports

over that logic into Djblets, so that other applications can make use of

the same API token capabilities.
This consists of backends for authentication, mixins for WebAPIResource

subclasses, an abstract model that projects can subclass for storing API

token data, policy validation and enforcement, and a model manager.
There are also unit tests for API token policy validation, plus a new

assertion function for helping test ValidationError messages.

Unit tests for both Djblets and Review Board pass (with the API token code

ripped out of Review Board).