maintenance: prune approvers monthly instead of weekly.

* Move approver pruning from a weekly task to a monthly task.
* Move weekly maintenance tasks from Wednesdays at 1:30am to Sundays at
  midnight (UTC).

lib/tasks/maintenance.rake

@@ -15,4 +15,9 @@ namespace :maintenance do
   task weekly: :environment do
     DanbooruMaintenance.weekly
   end
+
+  desc "Run monthly maintenance jobs"
+  task monthly: :environment do
+    DanbooruMaintenance.monthly
+  end
 end