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.

app/logical/sources/strategies/pixiv.rb

@@ -62,7 +62,7 @@ module Sources
       end
 
       def canonical_url
-        image_url
+        image_urls.first
       end
 
       def profile_url
@@ -122,7 +122,7 @@ module Sources
         tag.gsub(/\d+users入り\z/i, "")
       end
 
-      def download_file!(url = image_url)
+      def download_file!(url)
         file = super(url)
         file.frame_data = ugoira_frame_data if is_ugoira?
         file