uploads: switch to active job.

* Switch upload processing from DelayedJob to ActiveJob.
* Remove remaining references to delayed job from tests.

Closes #4128.

app/jobs/upload_preprocessor_delayed_start_job.rb

@@ -0,0 +1,8 @@
+class UploadPreprocessorDelayedStartJob < ApplicationJob
+  queue_as :default
+  queue_with_priority -1
+
+  def perform(source, referer_url, uploader)
+    UploadService::Preprocessor.new(source: source, referer_url: referer_url).delayed_start(uploader.id)
+  end
+end