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 for R18 posts, which are now supported
* replace the dead post for the imageMap test with an active one
* fix the ordering of images: it was possible to get the wrong image in
some cases when not using the batch bookmarklet, because of how fanbox
orders imageMap posts
* fix a bug where the sample wasn't properly replaced with the full
size
* rename test file to match the rest of the other files
