Fix doc generation for the extension resources
Review Request #9185 - Created Sept. 8, 2017 and submitted
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_featuresattribute due to that attribute not being on the
RBResourceMixinbut being on the
WebAPIResourcebase class. The
attribute has been moved to the mixin, fixing another crash.
With this patch applied, building the docs no longer crashes.