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/application_job.rb

@@ -3,15 +3,19 @@
 # The base class for all background jobs on Danbooru.
 #
 # @see https://guides.rubyonrails.org/active_job_basics.html
-# @see https://github.com/collectiveidea/delayed_job
+# @see https://github.com/bensheldon/good_job
 class ApplicationJob < ActiveJob::Base
+  class JobTimeoutError < StandardError; end
+
   queue_as :default
   queue_with_priority 0
 
   around_perform do |_job, block|
     CurrentUser.scoped(User.system, "127.0.0.1") do
       ApplicationRecord.without_timeout do
-        block.call
+        Timeout.timeout(8.hours, JobTimeoutError) do
+          block.call
+        end
       end
     end
   end