Fake article repository

I've extracted this out of #382 as it stands on it's own and it would be good to draw people's attention to it.

The gist is that it can be used as part of test setup to populate the article repository with known contents which can be helpful when then having to make assertions about how that content gets used.

Depending on your viewpoint this may be better or worse than working with real API interactions recorded & replayed by VCR though it proved useful for myself when having to develop a feature for which data wasn't yet available from the ``public_website`` API.

@andrewgarner @modsognir @jakubpawlowicz @gjvis