uploads: add new source:<url> dupe check (fix #3873)

* On the /uploads/new page, instead of just showing a "This post has
probably already been uploaded" message, show the actual thumbnails of
posts having the same source as what the user is trying to upload.

* Move the iqdb results section up top, beside the related posts section.

app/logical/sources/strategies/pixiv.rb

@@ -153,6 +153,10 @@ module Sources
         translated_tags
       end
 
+      def related_posts_search_query
+        illust_id.present? ? "pixiv:#{illust_id}" : "source:#{canonical_url}"
+      end
+
     public
 
       def image_urls_sub