Remove the `image_url` method from source strategies. This method would
return only the first image if a source had multiple images. The
`image_urls` method should be used instead. Tests were the main place
that still used `image_url` instead of `image_urls`.
Also make post replacements return an error if replacing with a source
that contains multiple images, instead of just blindly replacing the
post with the first image in the source.
Fix certain artist commentaries for foundation.app containing scrambled
characters. Apparently caused by the Nokogiri HTML5 parser not handling
UTF-8 input correctly when the encoding isn't explicitly set to UTF-8.
