Move database clean from cron to Sidekiq scheduler

After moving the remaining tasks from schedule.rb to sidekiq.yml, we can
remove whenever and won't rely on cron any more. That will simplify the
setup and migration to a new server.

app/jobs/rake_job.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+# Executes a rake task
+class RakeJob < ApplicationJob
+  def perform(task_string)
+    Rails.application.load_tasks if Rake::Task.tasks.empty?
+
+    Rake.application.invoke_task(task_string)
+  ensure
+    name, _args = Rake.application.parse_task_string(task_string)
+    Rake::Task[name].reenable
+  end
+end