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 the test suite failing when trying to run it in the default state
with no config file or API keys configured. Most source sites require
API keys or login credentials to be set in order to work. Skip these
tests when credentials aren't configured.
