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.lock

@@ -143,6 +143,8 @@ GEM
       thread_safe
     encryptor (3.0.0)
     erubis (2.7.0)
+    et-orbi (1.0.3)
+      tzinfo
     execjs (2.7.0)
     fabrication (2.16.1)
     faker (1.7.3)
@@ -251,7 +253,7 @@ GEM
     oj (3.0.5)
     openssl (2.0.3)
     orm_adapter (0.5.0)
-    ostatus2 (1.1.0)
+    ostatus2 (2.0.0)
       addressable (~> 2.4)
       http (~> 2.0)
       nokogiri (~> 1.6)
@@ -386,6 +388,8 @@ GEM
       unicode-display_width (~> 1.0, >= 1.0.1)
     ruby-oembed (0.12.0)
     ruby-progressbar (1.8.1)
+    rufus-scheduler (3.4.0)
+      et-orbi (~> 1.0)
     safe_yaml (1.0.4)
     sanitize (4.4.0)
       crass (~> 1.0.2)
@@ -396,6 +400,11 @@ GEM
       connection_pool (~> 2.2, >= 2.2.0)
       rack-protection (>= 1.5.0)
       redis (~> 3.3, >= 3.3.3)
+    sidekiq-scheduler (2.1.4)
+      redis (~> 3)
+      rufus-scheduler (~> 3.2)
+      sidekiq (>= 3)
+      tilt (>= 1.4.0)
     sidekiq-unique-jobs (5.0.7)
       sidekiq (>= 4.0, <= 6.0)
       thor (~> 0)
@@ -499,7 +508,7 @@ DEPENDENCIES
   microformats2
   nokogiri
   oj
-  ostatus2 (~> 1.1)
+  ostatus2 (~> 2.0)
   ox
   paperclip (~> 5.1)
   paperclip-av-transcoder
@@ -527,6 +536,7 @@ DEPENDENCIES
   ruby-oembed
   sanitize
   sidekiq
+  sidekiq-scheduler
   sidekiq-unique-jobs
   simple-navigation
   simple_form