The docs for the extension resource were still pointing at the old

Djblets extension resource instead of the one we provide in RB. This was

causing building the docs to crash.
In addition, our extension resource was not getting the

required_features attribute due to that attribute not being on the

RBResourceMixin but being on the WebAPIResource base class. The

attribute has been moved to the mixin, fixing another crash.

With this patch applied, building the docs no longer crashes.