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.
* Add README files to several directories in app/ giving a brief
overview of some parts of Danbooru's architecture.
* Add documentation for files in config/.