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/mastodon.rb

@@ -18,10 +18,6 @@ module Sources::Strategies
       end
     end
 
-    def image_url
-      image_urls.first
-    end
-
     def image_urls
       if parsed_url.image_url?
         [parsed_url.full_image_url]