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_uploads_job.rb

@@ -0,0 +1,7 @@
+# A job that runs hourly to delete all completed, stale, or failed uploads.
+# Spawned by {DanbooruMaintenance}.
+class PruneUploadsJob < ApplicationJob
+  def perform
+    Upload.prune!
+  end
+end