* Avoid using robohash for article cover images
If their service slows down, loading the cover image times out in
system tests.
Use a locally served image file instead.
* Specify expected image format for view object test
We were relying on the internals of Faker to return a robohash.org url
and checking that we included that in the cloudinary url.
Rather than relying on the default behavior, explicitly pass a
robohash url in the main image for this spec.
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