uploads: switch to active job.

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

Closes #4128.

test/test_helper.rb

@@ -122,6 +122,4 @@ class ActionDispatch::IntegrationTest
   end
 end
 
-Delayed::Worker.delay_jobs = false
-
 Rails.application.load_seed