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/services/subscribe_service.rb

@@ -5,15 +5,31 @@ class SubscribeService < BaseService
     account.secret = SecureRandom.hex
 
     subscription = account.subscription(api_subscription_url(account.id))
-    response = subscription.subscribe
+    response     = subscription.subscribe
 
-    unless response.successful?
+    if response_failed_permanently?(response)
+      # An error in the 4xx range (except for 429, which is rate limiting)
+      # means we're not allowed to subscribe. Fail and move on
       account.secret = ''
-      Rails.logger.debug "PuSH subscription request for #{account.acct} failed: #{response.message}"
+      account.save!
+    elsif response_successful?(response)
+      # Anything in the 2xx range means the subscription will be confirmed
+      # asynchronously, we've done what we needed to do
+      account.save!
+    else
+      # What's left is the 5xx range and 429, which means we need to retry
+      # at a later time. Fail loudly!
+      raise "Subscription attempt failed for #{account.acct} (#{account.hub_url}): HTTP #{response.code}"
     end
+  end
 
-    account.save!
-  rescue HTTP::Error, OpenSSL::SSL::SSLError
-    Rails.logger.debug "PuSH subscription request for #{account.acct} could not be made due to HTTP or SSL error"
+  private
+
+  def response_failed_permanently?(response)
+    response.code > 299 && response.code < 500 && response.code != 429
+  end
+
+  def response_successful?(response)
+    response.code > 199 && response.code < 300
   end
 end