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/null.rb

@@ -12,7 +12,7 @@ module Sources
       end
 
       def canonical_url
-        image_url
+        url
       end
 
       def normalize_for_source