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.

lib/tasks/danbooru.rake

@@ -15,8 +15,8 @@ namespace :danbooru do
 
   # Usage: bin/rails danbooru:reindex_iqdb
   #
-  # Schedules all posts to be reindexed in IQDB. Requires the delayed jobs
-  # worker (bin/delayed_job) to be running.
+  # Schedules all posts to be reindexed in IQDB. Requires the jobs
+  # worker (bin/good_job) to be running.
   desc "Reindex all posts in IQDB"
   task reindex_iqdb: :environment do
     Post.find_each do |post|