sources: remove image_url method from base strategy.

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.

test/unit/sources/moebooru_test.rb

@@ -6,7 +6,6 @@ module Sources
       site = Sources::Strategies.find(url)
 
       assert_equal(site_name, site.site_name)
-      assert_equal(image_url, site.image_url)
       assert_equal([image_url], site.image_urls)
       assert_equal(image_url, site.canonical_url)
       assert_equal(preview_url, site.preview_url)