Fix Sidekiq warnings about JSON serialization (#17381)

* Fix Sidekiq warnings about JSON serialization

This occurs on every symbol argument we pass, and every symbol key in hashes,
because Sidekiq expects strings instead.

See https://github.com/mperham/sidekiq/pull/5071

We do not need to change how workers parse their arguments because this has
not changed and we were already converting to symbols adequately or using
`with_indifferent_access`.

* Set Sidekiq to raise on unsafe arguments in test mode

In order to more easily catch issues that would produce warnings in production
code.

app/workers/scheduler/user_cleanup_scheduler.rb

@@ -29,7 +29,7 @@ class Scheduler::UserCleanupScheduler
   def clean_discarded_statuses!
     Status.discarded.where('deleted_at <= ?', 30.days.ago).find_in_batches do |statuses|
       RemovalWorker.push_bulk(statuses) do |status|
-        [status.id, { immediate: true }]
+        [status.id, { 'immediate' => true }]
       end
     end
   end