More robust PuSH subscription refreshes (#2799)

* Fix #2473 - Use sidekiq scheduler to refresh PuSH subscriptions instead of cron

Fix an issue where / in domain would raise exception in TagManager#normalize_domain

PuSH subscriptions refresh done in a round-robin way to avoid hammering a single
server's hub in sequence. Correct handling of failures/retries through Sidekiq (see
also #2613). Optimize Account#with_followers scope. Also, since subscriptions
are now delegated to Sidekiq jobs, an uncaught exception will not stop the entire
refreshing operation halfway through

Fix #2702 - Correct user agent header on outgoing http requests

* Add test for SubscribeService

* Extract #expiring_accounts into method

* Make mastodon:push:refresh no-op

* Queues are now defined in sidekiq.yml

* Queues are now in sidekiq.yml

app/workers/scheduler/subscriptions_scheduler.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+require 'sidekiq-scheduler'
+
+class Scheduler::SubscriptionsScheduler
+  include Sidekiq::Worker
+
+  def perform
+    Rails.logger.debug 'Queueing PuSH re-subscriptions'
+
+    expiring_accounts.pluck(:id) do |id|
+      Pubsubhubbub::SubscribeWorker.perform_async(id)
+    end
+  end
+
+  private
+
+  def expiring_accounts
+    Account.expiring(1.day.from_now).partitioned
+  end
+end