Previously the extension and validation resources did not inherit from

the API token mixin nor the OAuth2 token mixin, making them inaccessible

via these methods. This was because they were not using our base

resource class due to being special cases of resources from Djblets.
Now we have a new RBResourceMixin that includes all mixins that Review

Board resources should inherit from. Resources defined by Review Board

will inherit them through our base WebAPIResource, but this mixin will

also allow us to use all these mixins in resources we bring in from,

e.g., Djblets.
The new extension resource explicitly inherits from this mixin, as does

the validation resource, allowing them to be accessed via OAuth2 tokens

and API token.

Ran unit tests.