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/post_replacement_processor.rb

@@ -67,6 +67,9 @@ class PostReplacementProcessor
     strategy = Sources::Strategies.find(source_url, referer_url)
     raise NotImplementedError, "No login credentials configured for #{strategy.site_name}." unless strategy.class.enabled?
 
-    strategy.download_file!
+    image_urls = strategy.image_urls
+    raise "#{source_url} contains multiple images" if image_urls.size > 1
+
+    strategy.download_file!(image_urls.first)
   end
 end