jobs: switch from DelayedJob to GoodJob.

Switch the ActiveJob backend from DelayedJob to GoodJob. Differences:

* The job worker is run with `bin/good_job start` instead of `bin/delayed_job`.
* Jobs have an 8 hour timeout instead of a 4 hour timeout.
* Jobs don't automatically retry on failure.
* Finishing jobs are preserved and pruned after 7 days.

app/jobs/prune_jobs_job.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+# A job that runs daily to delete all stale jobs. Spawned by {DanbooruMaintenance}.
+class PruneJobsJob < ApplicationJob
+  def perform
+    GoodJob::ActiveJobJob.where("created_at < ?", 7.days.ago).destroy_all
+  end
+end