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/mastodon_test.rb

@@ -17,7 +17,7 @@ module Sources
       end
 
       should "get the image url" do
-        assert_equal("https://img.pawoo.net/media_attachments/files/000/128/953/original/4c0a06087b03343f.png", @site.image_url)
+        assert_equal(["https://img.pawoo.net/media_attachments/files/000/128/953/original/4c0a06087b03343f.png"], @site.image_urls)
       end
 
       should "get the commentary" do