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/base.rb

@@ -179,6 +179,18 @@ module Sources
         return {}
       end
 
+      # A search query that should return any posts that were previously
+      # uploaded from the same source. These may be duplicates, or they may be
+      # other posts from the same gallery.
+      def related_posts_search_query
+        "source:#{canonical_url}"
+      end
+
+      def related_posts(limit = 5)
+        CurrentUser.as_system { Post.tag_match(related_posts_search_query).paginate(1, limit: limit) }
+      end
+      memoize :related_posts
+
       def to_h
         return {
           :artist_name => artist_name,

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

app/logical/upload_service/controller_helper.rb

@@ -23,11 +23,5 @@ class UploadService
 
       return [upload]
     end
-
-    def self.batch(url, ref = nil)
-      if url
-        return Sources::Strategies.find(url, ref)
-      end
-    end
   end
 end