uploads: merge tags when a duplicate is uploaded (fix #3130).

Automatically merge tags when uploading a duplicate.

There are two cases:

* You try to upload an image, but it's already on Danbooru. In this case
  you'll be immediately redirected to the original post, before you
  can start tagging the upload.

* You're uploading an image, it wasn't a dupe when you first opened the
  upload page, but you got sniped while tagging it. In this case your tags
  will be merged with the original post, and you will be redirected to the
  original post.

There are a few corner cases:

* If you don't have permission to edit the original post, for example
  because it's banned or has a censored tag, then your tags won't be
  merged and will be silently ignored.

* Only the tags, rating, and parent ID will be merged. The source and
  artist commentary won't be merged. This is so that if an artist uploads
  the exact same file to multiple sites, the new source won't override
  the original source.

* Some tags might be contradictory. For example, the new post might
  be tagged translation_request, but the original post might already be
  translated. It's up to the user to fix these things afterwards.

test/functional/uploads_controller_test.rb

@@ -130,6 +130,14 @@ class UploadsControllerTest < ActionDispatch::IntegrationTest
 
         assert_response :success
       end
+
+      should "redirect a completed upload to the original post if it's a duplicate of an existing post" do
+        @upload = create(:completed_file_upload, uploader: @user)
+        @post = create(:post, md5: @upload.media_assets.first.md5, media_asset: @upload.media_assets.first)
+        get_auth upload_path(@upload), @user
+
+        assert_redirected_to @post
+      end
     end
 
     context "create action" do