maintenance: break maintenance tasks into individual jobs.

Break the hourly/daily/weekly/monthly maintenance tasks down into
individual delayed jobs. This way if one task fails, it won't prevent
other tasks from running. Also, jobs can be run in parallel, and can be
individually retried if they fail.

app/jobs/prune_posts_job.rb

@@ -0,0 +1,7 @@
+# A job that runs hourly to delete all expired pending, flagged, and appealed
+# posts. Spawned by {DanbooruMaintenance}.
+class PrunePostsJob < ApplicationJob
+  def perform
+    PostPruner.prune!
+  end
+end