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

Gemfile

@@ -35,7 +35,7 @@ gem 'link_header'
 gem 'local_time'
 gem 'nokogiri'
 gem 'oj'
-gem 'ostatus2', '~> 1.1'
+gem 'ostatus2', '~> 2.0'
 gem 'ox'
 gem 'rabl'
 gem 'rack-attack'
@@ -48,6 +48,7 @@ gem 'rqrcode'
 gem 'ruby-oembed', require: 'oembed'
 gem 'sanitize'
 gem 'sidekiq'
+gem 'sidekiq-scheduler'
 gem 'sidekiq-unique-jobs'
 gem 'simple-navigation'
 gem 'simple_form'